So I built up a Mach 2 and it rides pretty well, but I can't get rid of a whirring sound coming from the rear of the drive train. It's most noticeable in high gear, though I can hear it a bit through out the gear range. Guessing the noise is coming from the rear derailleur. Adjusted the B-tension on the SRAMx7 so that it could handle the largest cog on the 7-speed MegaRange cassette, without jumping gears on the smaller ones. Also adjusted the cable length so that the guide pulley aligns with each cog and the chain does not rub on either side. Could the problem be that the SRAM derailleur cage is too narrow for a 7-speed chain? I don't see any links touching against the inside, but must admit it is a pretty close fit. (Don't hear any noise in the drive chain of my DF with the same size MegaRange freewheel and a crappy Tourney derailleur, whose B-tension screw is merely cosmetic.) Incidentally, the rear fork was spread by the frame builder to accommodate a tandem wheel, can't tell if the chain line has anything to do with the noise. Wondering now if maybe the derailleur hanger could be bent or misaligned.
Any ideas, folks?