I have a 97 Cherokee Sport, 4.0 inline 6. It has a metallic knock in what appears/sounds like the top end (using mechanics stethoscope).
One moment it was quiet...the next it was knocking/tapping. It happened on the way home from work, which is only a mile from home. I drove it the rest of the way home and parked it. I thought it was a stuck lifter so I ran some lifter cleaner through it to no avail. I finally pulled the head and replaced the lifters and pushrods. I examined the head (valves, etc...) and the head looked pretty good. No deposits, burnt valves, springs looked good, cylinder heads looked VERY good. Looked in ports and the valve color was good. No scarring, burnt pistons, etc... I know I should have had the head tested or checked but I was trying to do as little possible to get it back on the road.
Even with the knock/tapping, the motor runs great. It doesn't skip. It doesn't smoke. The oil pressure holds good at about 42 lbs.
I was told it may be a badly worn valve guide causing the noise but I'm not sure I can believe that unless it had a bent valve, which should've been very apparent when I took the head off.
Any help or suggestions would be highly appreciated.