Engine Noise

Check for loose or worn drive belts.
Check oil level.
Check valve clearance. See ADJUSTMENTS - 4-CYL article in the ENGINE PERFORMANCE section.
Check engine compression. Check engine mechanical condition. Check intake manifold, manifold gasket, camshaft and valves.
Check engine control solenoids. See CIRCUIT TEST SCG in TESTS W/CODES - EEC 2.5L article in the ENGINE PERFORMANCE section.
Check for loose components.
Check for vacuum leaks.
Check air intake system. Also, ensure throttle plate and linkages operate properly.
Check for loose spark plugs.
Check for cooling system leaks.
Check Evaporative Emission (EVAP) system for leaks.
Check ignition system. See BASIC TESTING - 4-CYL article in the ENGINE PERFORMANCE section.
Check exhaust system for leaks.
Check EGR system for leaks.
Check for worn connecting rod bearings, worn main bearings, loose piston pins, excessive piston to cylinder bore clearance.
Check for detonation (spark knock).