2008 Chevy Cobalt.
Question: My daughters 2008 Chevy Cobalt is making a humming noise. The car sounds fine when
you turn on the ignition.  Sounds find until acceleration.  When you accelerate it starts humming
loudly like the engine is revving up or like it isn't shifting properly.

However the rpm's drop as they should when it shifts.  The car sounds fine at idle.  Sounds fine
when you put car into gear.  But during hard acceleration it starts to hum.  You can push down the
gas and it starts to hum and continues to hum when you let off of the accelerator.  Then it seems
once the car comes to a complete stop, still in gear and still on the humming stops but starts back
up again during acceleration.  Please help.

You need to figure out if the humming is engine related or vehicle speed related. Since it doe not do
it sitting still, you need to get the car moving with the engine off to see if it is related to the
transmission or other driveline components. Get up to a speed where the noise is very noticeable
then pop in it neutral and see if the noise changes or goes away.

If the noise stays the same, then a bad hub bearing is a common problem to cause a humming
noise. It would change pitch as the vehicle speed decrease or increase. It would not be effected by
engine speed.

Another thing bearings do is change pitch when turning. Drive at 40 MPH and do a swooping turn to
the left and right. Sort of like being on a highway on/off ramp.

You could also have bad
tires. This would be the same thing- effected by vehicle speed, not engine

If the noise is related to engine speed, have the motor mounts checked. If one is worn out, it will
cause the engine to vibrate on the frame.

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2008 Chevy Cobalt Humming Noise When Driving