Stock head pipes and knocking ?

[Scarem]

I have a 06 FLHTC, 95", Dewey's street heads, and 37g's.  Using the stock head pipes with Rush slip on's, I hear
a faint knock from up front. I swapped the pipes to Fulsac dual conversion and rush, knock is gone.
I thought it might have been the 3/4" intrusion at the Y pipe , so I ground that down but still knocked.
Is this a reversion knock?