nasty exhaust noise!

i have a 94 s10 4 cyl. w/ 77k. lately after driving a lil harder than before i have noticed when i shift above 2500 rpm i get a nasty sound like a piece of the header was gone or somethin. the reason i think its the header is becuause my friends go kart w/ out a exhaust manifold sounds the same way. my problem in only shifting tho. help please.