I'm not there to see the movement and feel the pressure required to generate it, but if the mount's intact and the pipe's not cracked or leaking then you're not likely to find a hum in the exhaust. Does the humming seem speed related, or RPM related?
At 70, tromp the throttle and at the different engine RPM did the hum change?
Ditto, but let off the gas - hum change?
Go 80, put it in neutral and coast down to 60, then put it back into Drive. Did the hum progress down with vehicle speed?
If your spiders/shafts are greased, then I'd speculate a tire issue, followed by a rear wheel bearing. The rear bearings are often ignored as few realize the 80 has it as part of the HD full floater rear end. How many miles on your rig and what's the service history there? As to tires, there will be no visual clues to them being the problem, but you can swap them front to rear to see if anything changes. Tires would be the next guess.
One wild guess from years of wheeling would be to examine your exhaust tip. If it's been distorted, or has an edge ground a bit from offroading or dragging against a curb, this can make a lot of noise at speed. If so, make the opening round and file off any rough edge. I've seen two identicall brand new cars go by the DOT's audio meters to pass the drive by noise requirement. One did not make it because the tools cutting the exhuast tip were dull that day. A few hits with a file and it was able to pass.
Doug
). There are 3 mounts on my '93.
