I have a 2008 fxd 3200 miles with V&H slip on pipes and Fuelpak.( mapping has been checked) When the bike is at an idol you hear a sound from the back (maybe from the pipes) that sounds like a marble being drooped in to a pale of water (plop) . My dealer hears the noise but has no idea what it is and thinks there is no harm to the bike. The bike will back fire some times when I let off the gas.
Any ideas

 

I cant remember where I read about this but there is a condition where the shock-wave from the cylinder's exhaust port reaches the end of the tailpipe @ the same instant that the exhaust valve is in the open position. Somehow it sends an echoed wave back up the pipe and into the open valve (geez that sounds kooky) The article described not only an odd sound but said that the turbulence somehow sucks (or blows) unspent fuel out of the combustion chamber causing a loss of horsepower.
I wonder if the unspent could cause the backfire.. It sounds silly but you might try loosening the muffler clamps and sliding the muffler slightly forward or back (slightly changing the overall length of the exhaust) I assume that would change the timing of that shockwave arriving back at the exhaust port.
Its free and weird enough to work.

 

Thanks I'll try any thing