Normally you can see the cracks (if their is one) with a good flash light and maybe a mirror. I find most of the cracks around the collector (where the 2 pipes come together above the flange at the bottom of the header). It will crack around the welds. Also check around the rear 5 and 6 cylinder tubes. Some will aslo crack at the bellows (if installed) off cylinder 1-2.
It's best to view from the drivers side and or under side of the engine.
With some cracks, the sound is more apparent when cold then when hot. This is due to heat expansion.
[FONT=Trebuchet MS]01' Jeep TJ (Jeep #5)
3.5 RE Custom SuperFlex, Raingler, Kilby, Currie, Banks, Borla, American Racing, Custom mods, Always more in the works.