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t h i s method, t h e noise most l i k e l y can only be iso-
lated b y r e m o v i n g the d r i v e shaft and operating t h e
engine w i t h t h e t r a n s m i s s i o n "in-gear".
TIRE NOISE
T i r e noise i s often m i s t a k e n for rear axle noise
even t h o u g h t h e noisy t i r e s are actually located o n
the front wheels. Tires t h a t are unbalanced, w o r n
unevenly or have surfaces o f non-skid type design, or
are w o r n i n a "saw-tooth" manner are usually noisy
and often produce a noise t h a t appears t o originate
i n t h e rear axle.
T i r e noise changes w i t h different road surfaces, b u t
rear axle noise does not. I n f l a t e a l l four tires w i t h
approximately 20 psi (138 kPa) more t h a n the recom-
mended i n f l a t i o n pressure (for test purposes only).
This w i l l alter noise caused b y tires, b u t w i l l not af-
fect noise caused b y t h e rear axle. Rear axle noise
usually ceases w h e n coasting a t speeds less t h a n 30
m p h (48 km/h); however, t i r e noise continues, b u t a t
a lower frequency, as t h e speed is reduced. Rear axle
noise usually w i l l change w h e n i t is compared d u r i n g
" d r i v e " and "coast" conditions, b u t t i r e noise remains
approximately t h e same.
Discern between t i r e noise and axle noise b y n o t i n g
i f t h e noise varies w i t h vehicle speed, or sudden ac-
celeration and deceleration. Exhaust and axle noise
w i l l change w i t h t h e vehicle speed w h i l e t i r e noise
w i l l r e m a i n constant a n d i s more pronounced a t
speeds o f 20 t o 30 m p h (32 t o 48 km/h). Test addi-
t i o n a l l y for t i r e noise b y d r i v i n g over smooth pave-
ments or d i r t roads (not gravel) w i t h t h e tires a t
n o r m a l i n f l a t i o n pressure. I f the noise is caused b y
the tires, i t w i l l noticeably change or disappear a n d '
reappear w i t h changes i n the road surface.
FRONT WHEEL BEARING NOISE
Loose or " r o u g h " f r o n t wheel bearings w i l l cause
noise t h a t can be confused w i t h rear axle noise; how-
ever, front wheel b e a r i n g noise (2WD or disengaged
4 W D ) does n o t change w h e n i t is compared d u r i n g
" d r i v e " and "coast" conditions.
L i g h t application o f the brake pedal w h i l e m a i n -
t a i n i n g constant speed w i l l often cause wheel bearing
noise t o d i m i n i s h because t h i s action w i l l
remove
some w e i g h t f r o m the bearings. F r o n t wheel bearings
can easily be tested for noise by r a i s i n g and support-
i n g t h e vehicle and " s p i n n i n g " the front wheels (also
by " s h a k i n g " t h e wheels t o determine i f the bearings
are loose).
GEAR NOISE (DRIVE PINION AND RING)
A b n o r m a l gear noise is rare and, i f i t exists, i t i s
u s u a l l y caused b y " s c o r i n g " o n t h e r i n g a n d drive
p i n i o n gears as a r e s u l t o f insufficient or incorrect l u -
b r i c a n t i n t h e d i f f e r e n t i a l housing. The differential
side gears a n d p i n i o n gears very seldom are damaged
because t h e y are only "loaded" w h e n the rear wheels
rotate a t different speeds (e.g., w h e n t h e vehicle is
t u r n i n g ) .
A b n o r m a l gear noise can be recognized easily be-
cause i t produces a " c y c l i n g " tone t h a t w i l l be very
pronounced w i t h i n t h e vehicle speed range t h a t i t oc-
curs. T h e noise can occur d u r i n g one or more of t h e
f o l l o w i n g d r i v i n g conditions:
• " d r i v e " ,
• " r o a d l o a d " ,
• " f l o a t " , o r
• "coast".
A b n o r m a l gear noise usually tends t o peak w i t h i n
a n a r r o w vehicle speed range or ranges and i s more
pronounced between 30 t o 40 m p h (48 t o 64 km/h)
and 50 t o 60 m p h (80-96 km/h). W h e n objectionable
gear noise occurs, note t h e d r i v i n g conditions and t h e
speed range.
Conduct a gear teeth contact p a t t e r n test t o deter-
mine i f t h e best possible gear teeth contact patterns
exist. I f t h e patterns are n o t acceptable, adjust t h e
r i n g gear a n d the drive pinion gear " m e s h i n g " to ob-
t a i n t h e best possible t e e t h patterns. I f the noise s t i l l
exists after t h e adjustment, replace the gears w i t h a
" m a t c h e d " replacement gear set.
BEARING NOISE (DRIVE PINION GEAR SHAFT AND
DIFFERENTIAL)
Defective o r damaged bearings w i l l n o r m a l l y pro-
duce a r o u g h " g r o w l " or g r a t i n g noise t h a t is con-
stant i n p i t c h a n d varies w i t h t h e speed o f t h e
vehicle. B e i n g aware o f t h i s w i l l enable a mechanic
to discern between b e a r i n g noise and gear noise.
D r i v e p i n i o n gear shaft bearing noise t h a t results
from defective or damaged bearings can usually be
identified b y i t s constant, rough sound. D r i v e p i n i o n
gear shaft f r o n t b e a r i n g noise is usually more pro-
nounced d u r i n g a "coast" condition, where as drive
p i n i o n gear shaft rear bearing noise i s more pro-
nounced d u r i n g a " d r i v e " condition. T h e drive p i n i o n
gear shaft bearings are r o t a t i n g a t a higher rate o f
speed t h a n either the differential side bearings or the
axle shaft bearings.
D i f f e r e n t i a l side b e a r i n g noise w i l l usually produce
a constant, r o u g h sound t h a t is m u c h lower i n fre-
quency t h a n t h e noise caused by drive p i n i o n gear
shaft bearings.
B e a r i n g noise can best be detected b y road t e s t i n g
the vehicle o n a smooth road (black top). However, i t
is r e l a t i v e l y easy t o mistake t i r e noise for b e a r i n g
noise. I f a doubt exists, the tire treads should be ex-
amined f o r t h e i r r e g u l a r i t i e s t h a t often produce a
noise t h a t resembles bearing noise.
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