Does it on any surface. Did not start till about 15k , never noticed it before that. Maybe the 880s? Front does not look cupped but the last 2 rear tires were cupped bad. New back tire did not eliminate last time but who knows. Anything can be done about tires cupping?

 

Tire cupping is from bad balancing. Do you have a laced rear rim? Could be out of round. Have it checked for trueness. And check tire pressure every time you ride.

 

Like others have indicated most likely tires but you can very easily eliminate muffler mounts as a cause by spraying some lube on the rubber bushings and going for a ride. You used the word sing to describe the sound some will describe the sound from muffler mounts as chirping.

 

And I thought I was hearing things, Wheww. Glad it ain't just me.

 

I have the same problem with my 09 SG. rear tire is about time to replace but have had it for the last 10,000 miles. Can't fell any cupping. If I look at the rear it looks flat in the middle? OME Dunlops. Got anothe rnew tire & rim ready to on after I get back from vacation. Not riding my bike renting one up in Maryland so I can ride with my brother. 200 bucks to fly up & 2 hour flight beats 12 hour riding each way & it would cost me 140 in gas up & back.