Amita,
per the previous post, if you have a std exhaust can, I'd be checking how it seals at the pipe, as with the std can it shouldn't do this. Even with a good seal at the pipe/can junction, it is still common for a/m cans to pop and shoot some blue flame out the exhaust on the over run, eg, closed throttle at reasonably high revs. This becomes more pronounced as you raise above sea level as the fuel mix gets richer in fuel and leaner in air, leaving more unburnt fuel to find its way to the end of the can and once it mates with some air at the end of the can, ignition, poping sound and blue flame.
It's no big deal unless your carb jetting is so out of whack it is doing it when you shut the throttle at 4000 rpm. I can kick my bike back a gear at 10,000 rpm at the track and I will just hear some soft poping coming from the can. However if I go say +3000ft above sea level and kick it back at 8000rpm, it lets out very, very loud booms until I get back on the throttle. A mate followed me one night up in the hills and he said each time it popped, a foot of blue flame would shoot out the exhaust.
Tony