Narayana, take the side covers off. With the bike running, run your hand over the exhaust and look for leaks (cracked or loose?). Don't grab it, it will be hot. You can grab your carb and see if it wiggles around and changes the pitch of the engine. Sounds like something is loose, to me. If you find it, either take it back to your mechanic or tighten it up yourself - just don't overdo it.