Nothing I can think of, hmmm, maybe the tach itself??
I guess if I were you I would get a set of cluster hooks and just pull the cluster out, just so it's loose from the dash itself, to see if there is a change.
If it still does it, and you still hear the noise, I think you can remove the connectors and remove the whole cluster, and I think everything should be OK to start the car and see if it is still doing it. Nothing coming to mind right now on what it could be.
Gilly
PS, pic of the hook tools in case you don't know what I mean
, there should be an article somewhere on how to make your own and how to use them.