Roush strut tower caps & thuds
 

Does anyone run the Roush (or any make I suppose) billet aluminum strut tower caps?
They're here: http://store.roushperformance.com/detail.aspx?ID=695

Though the write-up on the Roush site says they use double-sided tape, they are actually tapped and threaded to fit onto the strut bolts. They look good, but the problem is that with them on, they cause a constant thud or thunk over bumps. At first the big center cap would loosen up, then I used some rubber and a pair of channel locks to snug them down. They stayed tight, but the thunking over the bumps continued. In fact, I was starting to question if it was actually the caps or if I had a loose suspension component. Took them off to solve the 'mystery' and the thunks are gone. Anyone else experience this with strut caps? I'm going to Roam Depot to find some large rubber washers to try out, but looking for any advice from someone who's dealt with it before.