Starts up and runs like a top for a few seconds then develops a rattle. Its a 4.0. Oil pump maybe? Anyone else had this issue?

 

Anyone?

 

If the rattle is coming from the center of the front of the engine I would more likely suspect that the bearings on the water pump are starting to make noise. You can usually use a mechanics stethoscope to listen to the thermostat housing and even the coolant hoses coming off of it to confirm it is the water pump.

Is it a chirpy kind of rattle?

The water pump pulley should have no play in it. So next time you are by your Jeep with the motor off, put your hands on the pulleys and see if any of them have play.

Many things can rattle, but if it rattles while sitting in the driveway idling, the most common causes will be on the belt path somewhere. (not the only causes, just the most common).

 

Ill begin there. Just bought this thing for 800 bucks. Im new to Jeeps so i wanted to ask before pulling it apart.

 

I would poke your head under the Jeep and see if the exhaust is resting on the transmissions cross member. Common problem when the mount goes bad.

 

Sometimes the flex plate bolts need tightening to the torque converter.
My 89 has rattled for the last 400K km.