97 model F150 Romeo 4.6L with 175,000 miles, auto tranny. Ran very quiet until 6 months ago and I noticed a slight knock at idle which comes and goes about every 5 seconds. It will not knock for about 5 seconds then knocks for aobut 5 seconds. This reoccurs continuously. Never had a problem with cold engine piston slap. Oil has been changed every 3,000 miles, owned since new. This thing has been babied and never run hard!! You can put your hand on the valve covers and the intake manifold when it knocks and the knocks can be felt also. Good oil pressure. If you run it up to 2,000 rpm the knocks can still be felt, but not heard. The engine runs great. THe sound does not sound like a lifter tick, it's more of a knock, sounding most like a bongo drum. Seems like if it were bearings it would knock continually and not go away. I'm stumped. I hate to pull it if it doesn't get any worse, I'll keep on running it. Any ideas????

 

Try disconnecting one fuel injector at a time and listen for the noise to lessen or stop. If the noise changes than that cyl could be suspect. I am thinking you may have carbon buildup on the piston dome/domes. The noise should sound more like a tapping noise if it's a bearing and carbon would sound the same only a little more solid or heavy. Also heat tends to change the carbon issue which can make it come and go. You mention driving the truck easy so that leads me to the carbon as well. A bad lash adjuster tends to sound like a marble in a can and is erratic. I know some guys here have had knock issues that turned out to be catalytic converters. The bongo drum sound eluds me but it sounds like it may be exhaust related. I hope this helps a little. Maybe someone else can relate to the noise you are hearing. Good luck

 

Thank you dynotech. You've given some good ideas. I've pulled each plug wire off one at a time and no changes to the noise, it came and go even when each cylinder was under a misfire. It neither got louder or quieter. I'll drive this thing until it's bad enough to find for sure. Maybe it won't get any worse, if it goes all together, I'll drop in a reman. Thanks.

 

Finally found it. Took the belt off and noise went away. Put the belt back on, and did the big screwdriver to the ear trick and the alternator seemed to the culprit. Took the alternator off. When spinning the alternator, it had a slight noticable clunk that would sometimes repeat itself, sometimes wouldn't. Figuring it was the bearings, the shaft had very little play, I bought a new one and buttoned it all up. This had me stumped for around six months now. The noise is gone, thank God.