What part... the felt stickers?
I put the felt stickers on the end of the drive where the connectors are. The end of the drive that comes in contact with the side of the MacPro case. Every drive is a bit different with various thickness so you want to use the correct size felt pad. I found the felt pads at the local grocery store (Safeway) in the sewing/home supply section. It was a packet of various size thin round felt pads that you can peel off just like stickers. You might be able to find them at Radio shack or any hobby store. Since my drive is a fairly thick 500GB, I used the quarter inch stickers and stuck them directly on the drive just under the electronics. If your drive is thin, then use the smallest one or cut them down. Make sure that you don't buy thick felt pads or they may get in the way and your drive won't seat all the way in. If you're creative, you don't need to use the felt stickers. Just put something in there that will create a slight insulated gap where the drive meets the side of the computer. I'm not sure if it's even necessary to pad it if you do the screw insulators because I didn't bother testing if the pads made a difference of not. It just made sense to me that the drive might be transmitting the vibrations to the metal casing where the drive connects. I wanted to cover all my bases and it worked. Anyway, you could try just putting in a small piece of foam rubber or sponge as long as it's not too thick and is secured so it doesn't fall when shoving the drive back in.
thanks again for the detailed explanation.
the reason why i'm confused, is cuz i'm looking at my MP, and doesn't seem like the part under the electronic connector on the drive really touches the side of MP.
unless you're talking about the "blue squarish" portion next to the electrical connectors (in below pic)... then it'll make more sense. cuz under the connectors is the mobo, which is a bit recessed already.
but hey, if it's floating your boat, it's all good. i'll do the same with washers. thanks for the tip!
PS: pic from Powermax.