I've got the 2.5 liter, so not much help there. No lights and good oil pressure doesn't sound like anything catastrophic wrong. Get a chunk of tubing or a mechanics stethoscope and see if you can localize the noise. Lots of stuff will make noise under the hood if it's not happy. Distributors, belt tensioners, exhaust manifolds, air conditioning compressors. If the noise changes pitch or speeds up with increased engine RPM's, and the jeep is in neutral, focus on things attached to the engine. If it only happens when the Jeep is moving it may not be engine related.