I have a 2000 sportster with drag pipes. It made this same noise with the factory pipes though. I ended up figuring out that it is coming from the rear cyl sucking back in through the exhaust pipe. The bike runs like a raped ape so I don't think its a valve issue but I could be wrong. You can easily feel it if you put your hand in front of the pipe. Could this be from the big open pipes and I just need to baffle them? I'm thinking it's more prominent from the rear cyl because of the shorter pipe. Am I off or is this most likely the issue? I appreciate any advise.