Sounds like bad universal joints in the driveshaft or maybe the rearend has a lot of wear. You can check the driveshaft by shaking or turning it at each end. Just remember that sounds travel and maybe hard to trace. Jim
I concur with axeeon --if the u-joints are ok, chances are your idle is a little high. I have the same problem because I have to keep the idle somewhat high due to the fact I sometimes run the AC. As long as I've had the car, it has "thunked" into reverse.