I have a 75 280 with 4 speed tranny. It has a loud "buzzing" type of sound in 1st and 2nd, quits down a little in 3rd and pretty much disappears in 4th. It dosen't matter what RPM, and it sounds like its coming from the tunnel, but I'm not positive. Any ideas?

Buzzing is usually sheet metal -- usually a heat shield or something.

Can you feel any vibration through the shift lever and does it change with gear selection? Is there a clunk when going from reverse to forward and vice versa? Does it go away when the clutch is depressed? Have you done anything recently to the drive train? (Ujoints, oil change, tranny oil change {check tranny oil?}, clutch work, exhaust work, anything?) Sudden onset or been getting worse over time?

Bees can't fly faster than 50 mph.

Have you been underneath to look around? If it's RPM independent then everything from the tail end of the transmission back should be looked at.

Put it up on ramps and checked but nothing real obvious. No recent drive train work on my part, not sure what the PO did, but he said he hadn't done anything. There is a definite clunk when putting into reverse. no real vibration and yse it does stop when the clutch is in. I'm kinda stumped as where to begin.