my blue car has no hum at all, however I do have another 71 Grabber with a 411 in the stock 8" rear end. The gears just weren't setup properly for back lash. It has the same hum you are describing. I drove it like that about 10 years and it never got any worse. It went in storage after that for another 15 years...then Katrina . Anyway, I've got another complete 9", 389 gears, with a spool, going back in it when I do the restoration. I paid $250 for the complete rear end from someone on this board. Now I just gotta go get it.
My Maverick has a mini-spool and a set of 3.55 gears that we installed ourselves, and there is that same hum that scooper describes when I let off of the gas.
spool Mavman is correct,hum or howl is either pinion depth incorrect or pinion bearings worn and lose,pinion moves out on accel.and moves in on deaccel.thats why noise stops when you let off gas.To the original question run a LSD on the street,much easier on all your driveline parts,as everyone stated lots of pros and cons but in the long run its safer and easier on you driveline,SVO makes a repo of original Trac-Loc with billet case halves and hi-po clutches that works real well,I use it in a lot of my customers 8" & 9",they have an aggresive "will chatter" and a "smooth" style.
Air locker FTW!!! Well, that's be a hell of an idea if anyone made an air locker for the 8", but I don't think they do. Check out the detroit locker. If you really want a spool, but are worried about how it will drive on the street, then you want a locker. It's got spring loaded sprags that lock the axles together, unless you go around a corner or something and then it "pops" open so you can go around the corner better. If you want to mostly drive on the street, but still want to take it to the track some times, then get a clutch type LSD. Auburn made a nice LSD that used cones instead of plates. I don't know if you can get that for the 8" or not though. Anyway, those are real nice pieces.
