engine knock

I was just given a 1990 cherokee 4.0L with some problems, most are minor but one has me worried. The engine has a knocking sound coming from the lower end of the block. The knocking sound does not increase in volume when you rev the engine, it only increases in speed and the engine does not shake or vibrate. The knock occurs at idle and when driving. It runs quite smooth so I have a hard time believing its a bearing or rod about to go. I work next to a garage and the mechanics could not come up with a difinitive answer for the sound but a friend thinks it might be a lifter. Is there a way to figure this out without ripping the whole engine apart? Any other ideas for the knock?
Thanks

If it sounds almost like a clicking sound, its a lifter. Bearing noises often change with engine load as well as speed.
You may be able to eliminate it with an oil and filter change then add an oil addidive specifically designed to free up sticky lifters. Drive it for several hundred miles and see if it goes away.
Sometimes this works.

Other wise, a lifter may have collapsed, and would need replacement.
It might be possible a cam lobe has gone flat, in which case all the lifters and the cam would need replacement. However, flat cam lobes result in other problems like an engine miss and/or intake manifold backfires.