Rahulk, the 545i, 645i and 745i muffler has a valve that closes off one side. At a certain rpm or load level (not sure exactly) the valve opens up and allows exhaust to flow through both pipes. So at idle, it is normal for exhaust to come out only one pipe. The fact that you are getting a puff of smoke after idling for a time is an indication of worn seals.
Last edited by Stephen Max; 01-22-2013 at 08:23 AM.