1995 Dodge Caravan Loud thump

First off, thanks for this forum. The symtom I experience occurs when I start the car and drive off. I soon as I start hitting the brake pedal, even lightly, I hear and feel a loud thump from the rear of the van as if something heavy was being slamed into the back. This doesn't seem to affect actual brake performance, it still stops as I expect it to. I had the rear brakes looked at and was told they where fine.
The problem is not consistant, I got an oil change and the car was sitting at my mechanics lot, for a couple of hours. Upon start up, I tried to duplicate the problem but of course, no thump! It's been fine for a few weeks and now it's doing again.
Any ideas?