When the starter is activated the sprag clutch connects it to the crankshaft (simply speaking) turning it. When the engine starts, the crank now turns faster than the starter so the sprag slips allowing the starter motor to stop and not be ruined by spinning out of control. The sprag usually acts up when it slips all the time not allowing the engine to start (starter simply spins).
This sounds like a clutch slipping slightly. What oil are you using in the engine? How many miles on the clutch? You may have to pull the side cover and take a look. You will loose some oil though.
It also sounds like your back tire chirping on the pavement. Does it do it while the bike is running?
