My SM465 constantly makes noise.

[tucsonjwt]

I would not rule out the clutch and input shaft bearing. SM465 is a crude transmission but I am told it usually holds together for a long time. I just had my clutch replaced along with the input shaft bearing and an input shaft repair sleeve and the clutch fork retaining clip was broken off and caused some noise - not like what you describe but worn clutches can generate a lot of noise in the transmission area.