94 Auto, noise in reverse

My 1994 Grand Am, when in reverse, is making a bad vibrating sound. It is a very deep gutteral sound, and very hard to pin down, but it does go away when the car is in park, neutral or drive. (though it is noiser in park and neutral than drive.) Anyone had problems like this, or maybe know what it is? the car is a 3.1L with Automatic transmission. I recently had the car in for service on an unrelated (as far as I know) issue, and I also just replaced the alternator.

Thanks,
Matt

This may be too late ( I just signed up because I wanted to answere your question) but, I had the same problem and finaly found it was the motor mounts. I replaced the upper and lower, all was good.

Hope this helps if your still looking.

Z