I just removed my K & N permanent air filter for cleaning and reoiling and replaced it, temporarily, with a regular paper element filter (MANN). I'll go back to using the K&N soon( I've never had any problems with it or because of it).

Now, at certain engine RPMs when stepping fairly hard on the gas pedal there is a loud boomy exhaust noise which wasn't there before.

Anyone have any idea why?

make sure your air box is closed all the way, sounds like its not..