I'm back...It is an aluminum case, and there is no noise in neutral. The gear oil was changed a year ago when the new clutch went in. SAE 75W-90. (I forget what brand) It is still at the proper level. I guess I'll keep on drivin' it with two fingers crossed and see if it gets louder. It comes off the road in about 12 months to rebuild the engine and tranny.
Thanks everyone for your help!