One of my hard drives used to really make the side panels resonate on my pc101. I initially tried using bluetack around the edges of the side panel, and while this helped a bit, didn't completely stop it.
I've just managed to sort it out now by running a couple of strips of 10x4mm foam gasket along the inside of the side panels (in-line with the divider plate between the top and bottom sections). If you don't have any gasket, a few blobs of bluetack should do the same job.
This doesn't completely silence the seek noise, but it helps a lot.