Yesterday my 91 sierra started making a weird noise from the engine. Its a 350 tbi with a tbi performer intake, cam, and headers. The noise sounds almost like an exhaust leak and a rod knock mixed together. Theres been no change in performance but i shut it off after it had warmed up and tried to start it back up again and it just cranked but didnt turn over. Ive heard two possibilties besides exhaust leak and thats dropped valve seat or loose torque converter bolts. It isnt apparent at idle or when holding speed but when i accelerate, its apparent and the truck gets much louder. Any help is appreciated.

 

When you accelerate in park or neutral can you hear this noise or is it just when you accelerate in drive and reverse?