Rear headrest noise
 

My 1996 S500 Coupe has rear headrests that raise or lower by button controls on the consoles (front and back).
Anyway, when you push the button to raise the headrests, they go up silently and smoothly... once all the way up, there is a strong "thump, thump, thump" noise that occurs for about 3-4 seconds. It literally sounds like someone in the trunk knocking on the floor.
(Yep, I've looked in the trunk and nobody is there :-) )
When the head rests are lowered, they again operate smoothly and silently until they're all the way down.... then, its thump, thump again.
Any ideas???
Thanks, KenP