Grunting/Humming Noise in Reverse

[ZackN]

Hey Everyone,

I know Grunting/Humming may not be the most descriptive term for this noise, but I'm having trouble putting my finger on how to describe it. Quick run down: This is on a 2000 Pontiac Grand Prix w/ approximately 180k miles on it. This thing has run terrific for as long as I have had it.

Last week, I noticed that when I put it in reverse and gave it gas, it makes this very noticeable humming/grunting that almost sounds like vibration. This sound is only present while in reverse and giving is gas somewhere around 1500 RPM (the car does not have to be moving). No other problems or noises have been found during any other time, no slipping or hard shifting noted.

The sound started about 2 weeks ago, and was very intermittent, but is now there on nearly every back out.

Any idears?

Thanks guys!
Zack

[FordMan77]

What's the fluid look like? Wipe it on a white towel and note the color. Nice and red, or brown/black?

How many miles on this trans? When was the last fluid/filter change done? Is the fluid full?

[ZackN]

Fluid is red, and it is full. Trans has never been changed, so 180k. Not sure on the fluid and filter change...been a long time I'm sure.

[Trannyman95]

I would investigate the mounts and your downpipe if it is still stock. The inside of the stock downpipe has a sleeve that can break loose with age and when you put the car in reverse the engine rocks backwards from when going forward and can loosen it up and cause a rattle. Bad upper and lower mounts will also cause the engine and trans to move too much and can cause a vibration.

[ZackN]

Haven't checked the downpipe, but I did watch the engine as I put it in reverse and accelerated...Noticeable backward movement on shift, but it stayed pretty steady during acceleration (which is when the noise is heard)..It sounds like a low tone roar that is abnormal, accompanied by vibration felt in the cabin (not much, but a little).