Component dampening


I have a nice quality tube preamp, but the casework, when tapped or thumped on either the front panel or lid will emit a slight ringing tone. I would like to eliminate this by lining the interior of the front panel and lid with some type of sound deadening substance. Any suggestions on what type(s) of material would help me realize my goal would be appreciated. Thanks...
huron
3M makes some stuff, you can also get car audio padding for that purpose, like Dynamax (get Xtreme).

KP
Try adhesive-back polymeric damping sheets. Used for vibration control, on pipes, cars (under the hood, for example).
Available probably at local (large) h/ware store. I got mine from McMaster-Carr.
Cheap and VERY effective in my system! Cut to size and use on front-plate, top-plate, sides, caps... (BUT BE CAREFUL NOT TO TOUCH EXPOSED PARTS OF CAPS WHEN YOU OPEN THE MACHINE).
Cheers!