Noise is usually a very elusive problem, and is generally
not the fault of the transmission. Mechanics should road
test the vehicle to determine if the drivers complaint of
noise is actually in the transmission.
In numerous instances drivers have insisted noise was
coming from the transmission, investigations revealed it
was caused by one of the following conditions:
Fan out of balance or blades were bent.
Defective vibration dampers.
Crankshaft out of balance.
Flywheel out of balance.
Loose flywheel mounting bolts.
Rough engine idle producing rattle in gear train.
Clutch assembly out of balance.
Loose or broken engine mounts.
Power take-off was engaged.
Worn universal joints.
Driveshaft out of balance.
Universal joint angles out of plane or at
Center bearings in driveline dry, not mounted
Wheels out of balance.
Tire treads humming or vibrating at certain
Air leaks on suction side of induction system,
especially with turbo-chargers.
Mechanics should try to locate and eliminate noise by
means other than a transmission removal or an
overhaul. However, if the noise appears to be in the
transmission, try to determine what position the gear
shift lever is in when the noise occurs. If the noise is
evident in only one gear position, the problem is
generally traceable to the operating gears. Next, try to
break the noise down into the following classifications:
Growling, humming and grinding. These noises
are caused by worn, chipped, rough or cracked
gears. As gears continue to wear, the grinding
noise will be noticeable, particularly in the gear
position that throws the greatest load on the
A lack of lubricant or use of improper lubricant
can also result in growling and grinding noises.
This is because there is insufficient lubricant to
cool and cover the gears, which allows metal-to-
Hissing, thumping and bumping. Hissing noises
can be caused by bad bearings. As bearings
wear and retainers start to break up, etc., the
noise could change to a thumping or bumping.
Gear whine. This is usually caused by lack of
backlash between mating gears. Improper PTO
shimming is the big offender here.
Vibration. Todays improved highways mean
entire power trains are cruising at higher RPMs.
These higher speeds mean damage caused by
driveline vibration is more obvious than in the
When the maximum RPM of a shaft is reached,
it begins to bow. A resonant hum can be heard,
and a vibration will be set up. This type vibration
can cause gear seizures, broken synchronizer
pins, bearing failures, brinelling and corrosion.
During acceleration and deceleration, the shaft
may pass through half-critical vibration (half the
maximum RPM of the shaft). A whine or boom
may be heard at this point.
Metallic rattles. These noises within the
transmission usually result from a variety of
conditions. Engine torsional vibrations are
transmitted to the transmission through the
clutch. In heavy duty equipment, clutch discs
with vibration dampers are not used, so a rattle-
particularly in neutral-is common with diesel
In general, engine speeds should be 600 RPM or
above to eliminate objectionable rattles and
vibration during the idle. A defective or faulty
injector would cause a rough or lower idle speed,
and possibly a rattle in the transmission. A rattle
can also be caused by excessive backlash
transmission output gear.