You mentioned replacing all the bearings in the head....does that include spindle bearings and the back gear idler support bearings etc. ? How about the upper spindle support bearing at the top of the belt housing? The darned things have bearings all over the place and all it would take is one bad one to make that sort of noise. Now that I think of it you mentioned that if you disengage the spindle (move the back gear lever half way?) it's still noisy.
I would look for looseness of the pulleys on either end. Pulleys vibrating against either a drive or driven shaft are always a potential source of noise.
Edit-- you did mention a "slight rattle" due to the castellated interface, with regard to engaging and disengaging back gear, but it's important to keep in mind that as a source of noise at all times....not just when monkeying with the engagement/disengagement....at least it is on my VS head. A tiny bit of load (hand rubbing on spindle) will usually squelch it though. I noticed on disassembly that there is that sort of iron-oxide staine appearance to much of the castellated interface surface....the color that comes from constant rubbing of two ferrous pieces. I believe that there is some chatter in there at all times because even with new parts, it's not an extremely tight fit. It can't be or it would be cantankerous trying to get things to mesh.
Last edited by pcarpenter; 09-16-2009 at 11:24 AM.