Those Chevy/GM V6 3400 engines were notorious for that. I forgot that I had a '97 Venture that blew the intake gasket before it was out of warranty and did it again two years later. My wife had an 01 Malibu with the same engine and the same problemI guess i am lucky. No knocks rattles, or any other noises. Just burns a quart of oil a month. I have to add anti-freeze every few days too. I think both could be caused by seeping head gaskets.