1999 Oldsmobile Intrigue grinding/crackling noise

When I get in the car and begin to roll (backward or forward), I hear a grinding noise (sounds almost like knuckles cracking) which lasts for 1 second or less then stops. This happens mostly when the car has been sitting for a while (in the morning or after work), but also sometimes at stop signs, etc.. It happens whether I turn the wheel or roll in a straight line... Also, as a test I parked on a slight hill, placed it in neutral, let it roll backward, and heard the noise. (not sure if that eliminates a transmission problem or not)

It's been looked at by two different shops, neither of which could figure out the problem.

If anyone can tell me what the problem is or set me on the right path, I would be eternally grateful.

Thanks