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.
Answer:
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 speed.

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.
2008 Chevy Cobalt Humming Noise When Driving

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