I have a similar thing happening on my '10 FLHTCU. Last year it was fine as a 96" with TFI, AC and slip ons. This year as a 103" with 255 cams I have a noise when the engine starts to load up in the higher gears, sounds like little stones hitting the front fender. I have read through this forum and tried all the liitle things with no help. I think it is the compensator as when I stop the bike and I blip the throttle, there is a loud slap from the primary area. I have tried the auto chain adjustment procedure. I am dying to tear into the primary myself but my dealer assures me he will take care of the problem, he thinks it is either the compensator or it is pinging and will take care of whatever the problem is.
I am not trying to hijack this thread but my bike goes in 1st thing tomorrow so I will post the dealers findings maybe it will help the OP.