It could be caused by just about anything.
Assuming the noise you describe has a combustion source, you may be hearing unburned fuel igniting as it is evacuated from the cylinders.
To help ensure your combustion system is working properly, clean the throttle body and combustion chamber, run some fuel injector cleaner through the tank, replace your O2 sensors, and do a compression test.
If the noise continues after that, check the timing to ensure your timing chain hasn't stretched too much.
Black 1996 5.2L ZJ Limited