The foams I've read about aren't too effective. I use rope-caulk on the inside of the large panels to reduce noise and ringing - usually 10 or 15 (maybe a few more if it's a large panel) random lengths will deaden a panel pretty well. Only drawback is that it can make your PC heavy if you have to use a LOT.
. Back in my audiophile days it was used to prevent turntable platters, etc. from ringing.
.bh.