Sure, that's quite normal, for a number of reasons.
- Long duration cam profiles, holding the exhaust valve open while the piston is traveling down.
- Rapid cooling and shrinking of exhaust gasses.
- Reflecting sonic pulses, as demonstrated in this 2-stroke exhaust animation:
Of course, it can also indicate a problem, or be an opportunity for a scam.
Back in the olde daze, a mechanic might hold his shoprag against a car's exhaust outlet, and alert the owner that the rag is occasionally sucked back, claiming that he has a leaky exhaust valve, and sell him a top-end overhaul. The occasional suck-back was usually caused by a simple cylinder misfire, fouling plug, carb mixture problem, ...etc., normal for the engine, or fixed by a tune up, or a genuine mechanical problem.
Because of all these factors, the exhaust "suck-back" isn't a very good definitive diagnostic.
The compression and/or leak down tests are much better diagnostics.
So, if the bike runs fine, it's not something to worry about...
