I have a 1997 2.5 jeep wrangler.when you first start the engine it runs and sounds fine, then after about 20 seconds it begins to have a knocking sound only at idle. when you press the gas you cant hear the noise but when you let off it comes back. when driving you cant hear the noise unless you stop and idle. if you let off the gas quickly you hear a slight rattling then it begins the knock again at idle.

It may be piston slap. This is caused by either engine wear with the piston bores being too large for the pistons, or by poor manufacturing tolerances, or a combination of both.

To test for this, start the engine up first thing in the morning and run it for about thirty seconds while you listen carefully and memorise the sound and intensity. Turn the engine off quickly, remove the spark plugs and put two squirts of motor oil into each cylinder. Reinstall the plugs, start the engine up again and listen. If you have piston slap the noise will have been greatly reduced or even eliminated for about thirty seconds seconds and then returns.

The fix is to rebuild or replace the engine, but if it works just fine then don't worry. Many slappy engines keep going for tens of thousands more mile. The fact that your knocking is only at idle suggests your crankshaft is not to blame.