Chevrolet Cobalt Service & Repair Manual: Noise, Diagnosis
| A visual inspection must always
be performed to confirm the presence of aftermarket components, potential
collision damage, or areas of concern. |
| A noise produced by a vibration
can be diagnosed in the same manner as a vibration. A noise produced by
wind, leaks, or vehicle turbulence requires the use of special listening
tools. |
| When a vibration produces a noise,
the vibration can be transmitted from the originator, through the conductor,
and on to the reactor where the noise is heard by the customer. The noise
is transferred directly from component to component. For example, if the
steering system (originator) is producing an abnormal vibration, the suspension
system (conductor) can transmit the vibration to the instrument panel (reactor)
where a noticeable squeak is heard. |
| To be able to isolate a noise
to a specific system/component group, use one or more of the following noise
detection tools: Chassis ears, mechanics stethoscope, screwdriver, rubber
hose, and a ultrasonic leak detector. |
| Noises that are produced by wind,
turbulence, and air leaks are independent from noises produced by moving
components. The noise is not transferred directly from component to component. |
| Once a symptom is classified as
a noise concern, the particular conditions under which the noise occurs
need to be identified. These conditions are identified and verified on the
road test. |
| Detailed information from the
customer is key is duplicating and diagnosing a noise concern. Use of a
customer interview sheet can assist in pinpointing the origin of the noise
concern, Fig. 1. |
| Refer to Fig. 2
for noise concern diagnostic flow chart. |
| 1. |
Perform a vehicle road test and duplicate
noise concern. |
| 2. |
After identifying possible systems,
a preliminary inspection of these systems should be done. If cause
is identified at this stage, perform required repairs. |
| 3. |
If source of noise is unidentified,
use a listening device such as a chassis ear to pinpoint source
of noise. |
| 4. |
Once source of noise has been identified
using a listening device, determine if this source is related to
suspected system previously identified. If it is unrelated, then
it is possible that source of noise is a reactor to a noise being
transmitted through a transfer path. |
| 5. |
Common vehicle noises and applicable
vehicle and component systems that can be affected are as follows: |
| |
a. |
Grunt on acceleration: A raspy sound
and momentary vibration in floor pan during acceleration or following
a braking stop. Grunt on acceleration occurs on light trucks, and
is an concern of driveline system. Vehicle speed range is 0-2 mph. |
| |
b. |
Tip-in moan: A moan noise with a possible
vibration in floor during light acceleration. It is usually worse
at one particular throttle setting during acceleration at that speed.
Common vehicle speeds range between 25 and 50 mph. Tip-in moan is
possible on all vehicle types, and is an concern of engine and exhaust
systems. |
| |
c. |
Brake moan: A low-pitch noise that
is felt in floor and/or steering wheel during light brake application.
Common vehicle speeds range between 8 and 40 mph. Brake moan is
possible on all vehicle types and is an concern of brake system. |
| |
d. |
Hum or boom: These are low-pitch noises
often accompanied by a vibration felt in floor pan. Usually related
to driveline angle or balance. It may or may not be torque related.
Hum or boom occurs on all rear wheel drive vehicles and is an concern
of driveline system. |
| |
e. |
Howl: Howl is a medium-pitch noise
that may be heard at any speed. Typically, it can be affected by
acceleration or deceleration. Howl is possible on all vehicle types,
and is an concern of drive axles. |
| |
f. |
Whine: A high-pitch noise that may
be heard at any speed. It is typically, but not always, unaffected
by accelerating and decelerating. Whine is possible on all vehicle
types, and is an concern of drive axles. |
| |
g. |
Driveline clunk: A loud noise that
can be attributed to total powertrain response to torque reversals
in system, including, but not limited to, backlash in transmission,
driveshaft, and axle. Driveline clunk is possible on all vehicle
types, and is an concern of driveline system on rear wheel drive,
or drive axles on front and rear wheel drive vehicles. |
| |
h. |
Chuckle: Low-pitch noise that normally
occurs while decelerating to a stop; pitch goes down as vehicle
slows. When very loud, often described as a knock. Chuckle is possible
on all vehicle types, and is an concern of drive axles. |
| |
i. |
Rumble: Low-pitch noise that often
is loudest during turns. Can usually be identified as front or rear
in origin. Rumble is possible on all vehicle types, and is an concern
of drive axles. |
| |
j. |
Brake rattle: Noise of varying pitch
that changes as brakes are applied or released. Brake rattle is
possible on all vehicle types, and is an concern of brake system. |
| |
k. |
Chatter: Pronounced noise that occurs
only when turning. Chatter is possible on all rear wheel drive vehicles,
and is an concern of drive axle. |
|
Fig. 1 Customer interview sheet (Part 1 of 2)
Fig. 2 Noise diagnostic flow chart
Front
1.
Raise and support vehicle, then remove
tire and wheel assembly.
2.
Remove ball stud to steering knuckle
pinch bolt, then ...
1.
Remove power steering reservoir and
disconnect return hose.
2.
Start engine and drain fluid into a
suitable container.
...
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