2004 suzuki marauder

knock knock ? whats there... I just bought a 2004 Marauder 1600cc in almost showroom condition,2500mi.from a used car lot but with the low low miles and new,garaged appearance I figured no worries.The bike runs and handles perfect but, and here we go! Second gear & up under heavy load on throttle from 2000-3000 rpm there is a loud clattering knock that sounds like rockers the more throttle on this low rpm load the louder it is.I thought mabe it was octane so the first few fillups were just premium & i tried adding boost i think it helped slightly but its still there   after asking a local suzuki dealers mecanic to ride it to see what he thought,he said he wasnt sure it had been so long since he had worked on one he couldnt say if it was normal.He sugested test riding another bike & see if had it too. Today i rode a mean streak with aftermarket straight pipes, other than exaust i didnt hear a thing! HELP!!!!!!