Check the rubber iso-mounts at the front of the muffler. I've never ridden with them dmaged to know what it would sound like, but when I was disassembling my bikes to build one the first time, I found one of the mufflers had both iso-mounts torn in two and the mounting hardware was loose.
rxcrider was right. On my homeward commute tonight, I picked up a Mobil 1 M1-102 oil filter and a couple of liters for replacement. New filter didn't fix the noise, so further inspection found the rubber grommet (# 34321234629), holding the weight of the exhaust, working itself out of it's bracket. Apparently when new, it's held in by a circlip (# 07129934330), which is now missing. I reseated the grommet between the bracket and the exhaust holding arm. It was quiet around the block. Nice!
I'm going to order both pieces tomorrow.
1999 BMW R1100RT
2010 Harley Davidson ElectraGlide Limited