Okay... Its a 96 cherokee 4.0 6 cyl, 4wd, 4 door... when i start the jeep, you start to hear a rattling noise, definitely coming from somewhere around one of the pulleys or bearing of some sort. When given gas, the rattle gets fast, i assume because whatever it is is spinning faster, so last night I replaced the idler pulley; didn't stop... after puttin the stethoscope up to each of the pulleys. it was way louder on the water pump than any of the others. (there is an ounce of play in the water pump pulley when applied pressure) so i bought a water pump last night, putting it in later today... Does this seem like the ride idea? and if not, i have no clue what else it could be...

 

It sounds like you diagnosed it right. The water pump could definately make that type of noise. But just to be sure, check the harmonic balancer!

 

thanks... and the harmonic balancer would be....

 

The harmonic balancer is the bottom-most "pulley"

One other thing to check is what is known as the "idler pulley"

This one is the top-most. It runs NOTHING. It is just there as a place holder so the orientaion of the serpentine belt works.

All it is is a pulley and a bearing.
It is one of those parts that tends to "go" alot on Cherokees

Very easy to replace. Just loosen up belt (Have to loosen the power steering pump..the adjuster is the bottom bolt) Then just unbolt, install new and reverse everything you've done.

To test it, remove the belt and give it a spin.
Should spin freely with just a hint of resistance.
Place forefinger on the front of the pulley and rotate it with said finger...

You should "feel" NOTHING but a smooth spin.

If its jerky or you can actually hear the bearings....replace it.