97 low speed rumble

I don't know exactly how to describe the noise. When I lift off the throttle at 30mph, I hear a distinct rumble from the rear of the cabin. Once I give it throttle again the noise disappears?
I'm thinking it's related to the u-joints, since it only happen when power is not applied (coasting)?
Anyone else have a similar problem or know what it is?
Thanks,
Will

If you havn't already, check to see if they are loose. They can make noise and not be loose, but you still need to check them.

Does it make the noise in neutral?