I have A 96 cherokee 4.0 5 spd and it seems to have some kind of slap when im under load or say i put it in 3rd doin about 25 and give it gas. Piston slap? Timing chain? Someone help... I do have a cracked header in the common spot. Could it be slappin from no habin the right back pressure or what help please. Thank you.

 

Exhaust noises can sound like anything from clunking & popping to whistling bird noises. If you think the header's leakin' at the crack, i'd look there first.