gee...pretty good archiveist Robin.
Jason - is it the shifter arm vibrating? If so, you might be able to quickly fix it by checking to make sure the rubber shifter boot isn't pinching the arm. This is the boot under the leather decorative one.
It can sometimes pinch and pressure the shifter lever such that is vibrates.
Or, it would be wear in the tranny itself. My last tranny just vibrated the arm every now and then. If you pushed in the clutch and carefully adjusted the arm a bit without pulling it out of gear, the vibration would stop.
Never hurts to rebuild your shifter linkage. You'll like that whatever the issue is.
Cheers!
Jeff
Bellevue WA
90 535iM - not much stock remains. 3.7 liters, ported head, cammed, 3.73 diffy, M5 brakes, MAFed, yadda yadda yadda
86 Porsche 951 - Track Toy