Could very well be feedback in more than one form or another. Leo had a real knack for running wires a certain way and it could certainly be your problem also or a bad connection somewhere. Check all of your solder joints real well and make sure they are ok. Most will say it's filter caps as motor-boating is caused from insufficient filtering but IMO that's not the case. Heater wires not cancelling will do it to but usually in a buzz form. I re-did this amp a few months back that was doing the same thing except it was also high freq. oscillating. I even called Bruce and we went over the legistics but heres what finally fixed it. I put a 500pf cap from the treble wiper (pin 2) to ground and that stopped the farting. Then I moved the wire from the phase inverter off the coupling cap that goes to the grid resistors of the power tubes and routed it around the first preamp tube socket and all the noise went away and everything was good. You would be surprised what moving wires around can do in amps like this but it can be dangerous when idling and moving wires. Try finding pictures of a blaceface Deluxe board or a wiring diagram and make sure they are run the same way.