Strange rear end sound

For the last couple of months I have had an intermittent sound coming from the general area of the rear of the car at highway speeds(sometimes it lasts for a few seconds other times for half an hour). It is a very strange sound, almost like something is tied around the driveshaft and is slapping the bottom of the car (I have looked and can't find anything). The sound is rythmic (tah, tah, tah, tah...) at a rate of 3-5 beats to second. I am beginning to think it may be something inside the differential, I don't know what it could be there though, does anyone have any thoughts?

Ike

p.s. I did have an Aluminum driveshaft installed on the car a couple of months before all this started.

I have checked some of those(brakes, U-joint, but not the pinion flange bolts), I will go through and check the rest, although this does not have the usualy rubbing its more of a distinct "thwap", also it is not very loud (it can barely be heard over the stock radio at mid volume levels). One other strange thing is nothing apparent seems to trigger this, driving down a straight reasonably smooth road at constant speed this sound may start and stop at random, or it may go for a week without a sound.

Thanks Ike