Generator Vibration and Abnormal Noise: Causes and Site Checks

Who this page is for: facility and maintenance engineers, apartment and campus managers, and site supervisors who have noticed a set that “sounds different”, shakes more, cracks its own brackets or makes the floor or canopy buzz — and who want to know what to check before it becomes a breakdown.

Key facts

• A 1,500 rpm set turns at 25 revolutions per second (25 Hz). Anything that happens once per turn — imbalance, misalignment, a bent shaft — shows up at 25 Hz.
• A four-stroke engine fires each cylinder once every two turns, so the firing frequency is 12.5 Hz × number of cylinders: 50 Hz for a four-cylinder engine, 75 Hz for a six-cylinder engine, 150 Hz for a twelve-cylinder engine.
• A weak or misfiring cylinder adds energy at 12.5 Hz (half-order) — the cam-shaft speed. A rhythmic “lope” at roughly 12 beats a second is the classic sign.
• The alternator’s magnetic hum sits at 100 Hz (twice the 50 Hz supply frequency). A louder hum under load usually means unbalanced phases or a winding problem, not a mechanical fault.
• The useful measurement is vibration velocity in mm/s rms, taken at fixed points. The trend against the commissioning baseline matters more than any single number.
• Typical field rule: +25% over baseline — watch; +50% — investigate this week; double the baseline — plan a shutdown; sudden step changes — stop and inspect.

Why a generator vibrates at all

A reciprocating diesel engine converts explosive pressure pulses into rotation. Every power stroke kicks the crankshaft; every piston reverses direction twice a turn. Even a perfectly built engine produces firing pulses and unbalanced secondary forces, which is why a generator stands on anti-vibration (AV) mounts and why makers accept higher vibration on a genset than on an electric motor of the same size. The alternator adds its own small contributions: rotor residual imbalance, magnetic pull between rotor and stator, and bearing noise.

A healthy set therefore has a signature: a strong component at firing frequency, a smaller one at 25 Hz, a magnetic line at 100 Hz, and some broadband engine noise. Faults do not usually create vibration from nothing; they change the signature. A loose mount raises everything. A misaligned coupling raises 25 Hz and 50 Hz, especially in the axial direction. A failing bearing adds high-frequency noise that a velocity meter barely sees but a stethoscope or an acceleration reading picks up clearly.

The six sources, and how each one presents

1. Anti-vibration mounts and foundation

AV mounts are rubber or spring elements sized for the set’s weight and its lowest exciting frequency. Rubber hardens and cracks with heat, diesel and lubricating oil splash; in Hyderabad canopies that sit in direct sun, mounts age noticeably faster than the maker’s nominal life. A collapsed mount lets one corner sag, the set rocks, and vibration rises in every direction at once. A mount that has bottomed out — metal touching metal — transmits the full engine shake into the base frame and floor, and the floor itself starts to drum.

Foundation faults look similar: a cracked plinth, grout washed out after monsoon flooding, or holding-down bolts that have worked loose. If the set was recently relocated or a new plinth cast, suspect this first.

2. Coupling and alignment

Most sets above about 20 kVA use a single-bearing alternator bolted to the engine flywheel housing through flexible discs, so alignment is set by the machined faces. Problems appear when the flywheel housing bolts loosen, when the coupling discs crack, or on two-bearing sets where the alternator has been removed and refitted without proper laser or dial alignment. Misalignment shows as raised vibration at 25 Hz and 50 Hz, high axial readings at the alternator drive-end bearing, and heat at the coupling. Cracked coupling discs often give a knock on start-up and shut-down as the load reverses.

3. Alternator bearings

The non-drive-end (NDE) bearing of a single-bearing alternator carries the rotor weight and runs for thousands of hours on grease that slowly degrades. Early damage is a faint rumble or a regular “tick”; late damage is a growl, heat above about 80 °C on the bearing housing, and eventually rotor contact with the stator. Rising vibration at the NDE housing, with the rest of the set unchanged, points here. See alternator service and alternator bearings and torsional behaviour.

4. Engine firing and balance

A dirty or worn injector, a sticking valve, low compression on one cylinder or air in one fuel line produces uneven firing. The set “lopes” at half-order, the exhaust note becomes irregular, black or white smoke may appear, and the governor hunts slightly as it corrects each weak stroke. Uneven firing is a combustion problem showing up as vibration, so it belongs with the black smoke, white smoke and hunting diagnostics. Damaged torsional dampers on larger engines raise noise and vibration across a band of speeds and can crack crankshafts; that is a stop-and-inspect finding.

5. Canopy, panels and sheet metal

Many “vibration” complaints in apartments and offices are actually resonance: a canopy panel, louvre, door or cable tray whose natural frequency happens to sit near 50, 75 or 100 Hz. It buzzes loudly while the set itself is fine. Press a hand flat on the suspect panel while the set runs; if the buzz stops, you have found it. Missing fasteners, worn door gaskets and acoustic foam that has come unstuck are common after a few summers. See acoustic engineering for how enclosures are designed.

6. Exhaust system and supports

An exhaust pipe rigidly fixed to a building wall, without a flexible bellows at the engine outlet, carries engine vibration straight into the structure and cracks manifolds and turbo housings. A broken bellows also changes the exhaust note and can leak hot gas into the canopy. A booming low-frequency noise in an adjacent room usually means the exhaust or its supports, not the engine. See exhaust back-pressure.

Diagnostic table: symptom to most likely cause

How to measure it properly

A pocket vibration meter that reads velocity in mm/s rms is enough for most sites. What matters is repeatability.

1. Fix the points. Mark them with paint: engine front, engine rear (flywheel housing), alternator drive end, alternator non-drive end, and the base frame at each corner above and below the AV mounts. At each bearing point, read horizontal, vertical and axial.
2. Fix the condition. Rated speed, coolant at normal temperature, and a stated load — ideally the same load each time (for example 50% on a load bank, or “normal building load at 11 am”).
3. Record the baseline at commissioning or after a major service. Without it, you are comparing against a guess.
4. Read above and below each mount. A healthy mount reduces vibration sharply between the base frame and the floor. If the two readings are nearly the same, the mount is no longer isolating.
5. Find the frequency. A meter with a simple spectrum function, or a phone vibration app laid on the frame, will show whether the energy sits at 12.5, 25, 50, 75 or 100 Hz. Frequency points to the source; amplitude tells you how urgent it is.

As indicative guidance only: a rise to 1.25 times the baseline at any point is a watch item; 1.5 times is an investigation; twice the baseline, or any sudden step change, calls for a planned shutdown and inspection. ISO 8528-9 sets out how genset vibration should be measured and gives broad evaluation values by set size, but the maker’s acceptance figure for your model always takes precedence over these ratios.

Common mistakes

• Replacing parts before measuring. Bearings and mounts get changed on suspicion, the noise stays, and the real cause — often a loose panel or a bridged mount — is found later.
• No baseline. Without commissioning readings, nobody can say whether 15 mm/s is normal for this set or a warning.
• Tightening AV mounts solid. Over-tightening the centre bolt, or adding a washer stack, defeats the rubber and sends vibration into the building.
• Rigidly fixing the exhaust to the wall to “stop it rattling”, which then cracks the manifold or turbo housing.
• Ignoring the half-order lope. Uneven firing is treated as a mounting problem, when the cure is an injector or valve service.
• Running a set with a known growl from the alternator “until the next service”. Bearing failures accelerate quickly once they start.

What to send us

To diagnose from a distance, or to arrive with the right parts, send: the set’s kVA, make and model of engine and alternator, running hours, and how long ago the change was noticed; whether the set was recently moved, serviced or had a part replaced; a 30-second video with sound taken from each side of the set at normal load; photographs of all four AV mounts and the exhaust connection; and any vibration readings you have. Use the service and AMC enquiry, or WhatsApp the details and video to 98852 01608. Our service engineers attend across Telangana, Andhra Pradesh and every district within about 300 km of Hyderabad, with project support across India.

Questions engineers ask

How much vibration is normal for a diesel generator?

It depends on the engine, alternator, mounting and load, which is why the maker’s figure for your model and your own commissioning baseline are the references. As a field rule, a rise of 25% over baseline is a watch item, 50% calls for investigation, and double the baseline or any sudden jump calls for a shutdown and inspection.

How do I tell a mount problem from an engine problem?

Read vibration on the base frame above each mount and on the floor below it. A healthy mount reduces vibration sharply. If a reading below the mount is nearly as high as above it, that mount is not isolating. Engine problems change the spectrum, especially at half-order and firing frequency, rather than raising everything evenly.

Why does my generator lope or beat rhythmically?

A rhythmic lope at about 12 beats a second on a 1,500 rpm set usually means one cylinder is firing weakly, from an injector, valve or compression problem, or air in one fuel line. The governor corrects each weak stroke, so speed hunts slightly too.

What does a loud hum from the alternator mean?

A 100 Hz hum is magnetic and normal at some level. If it rises sharply with load or changes between phases, check phase currents for unbalance and have the windings and insulation tested. A mechanical growl or rumble from the rear of the alternator is different and points to the non-drive-end bearing.

Can canopy noise be fixed without touching the engine?

Often, yes. Many complaints are panel or louvre resonance. Missing fasteners, worn door gaskets, loose acoustic foam and unsupported cable trays are fixed in hours. Press a hand on a buzzing panel while the set runs; if the buzz stops, that panel needs stiffening or securing.

How often should vibration be checked?

Take a baseline at commissioning or after a major service, then read the same points at every quarterly service and after any relocation, repair or unusual event such as a short circuit or flooding. Readings mean most as a trend.

Is it safe to keep running a set that vibrates more than usual?

A gradual, modest rise can be run while an inspection is planned. A sudden step change, a knock, a metallic scrape or a hot bearing housing is a stop condition. Coupling and bearing failures can escalate within minutes.

Related pages

• Generator troubleshooting
• Generator hunting: speed, frequency and voltage swings
• Alternator bearings and torsional behaviour
• Alternator service
• Acoustic engineering
• Exhaust back-pressure
• DG set foundation and anti-vibration mounting
• Preventive maintenance

Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 5 November 2026.