It's called the TWEETY!....
Why does my bike sound like an old VW?
This is affectionately known as "tweety," and can happen to bikes anytime after a few thousand miles. It seems to appear earlier on bikes that are ridden on a lot of short trips, thereby allowing condensation to remain in the muffler. It also seems to be traced to clogging of the spark arrestor system. The noise is merely an annoyance; it will not damage your engine. Aside from replacing the exhaust system, you can try to fix it:
Take a huge straightblade screwdriver (like 12" shaft), and sharpen the end with a file. Look in the end of the exhaust, there is a small perforated pipe running from the opening to about 8" inside. It has a seam on the bottom (6:00 position) that may already be split open. Using the screwdriver and a hammer, drive the screwdriver into and along the seam, opening it up to within 1" of the end. Start the motor and listen for tweety. If it's still there, make another lengthwise slit at the 12:00 position, start the motor and check. Some people have had to make four slits (at 90 degree intervals) to fix the tweet.
Some bikes get it some don't.
Heres a link to a possible fix. http://www.klrforum.com/showthread.php?t=1985
It took about 70% of the tweet out of my buddies pipe. Of course we are running Jardine pipes now, so no more tweety anyway..