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????

 

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.

 

Was gonna say if it was just a slight knock, use Marvel Mystery Oil in it the next oil change. Ive known numerous mechanics that swear by this stuff. Ive put it in engines that had terrible upper and lower end knocks. After about 10 minutes total silence.

 

MMO is the best stuff in the world! I have always used it in my high mileage engines, and my last truck went 250k, before I sold it 2 years ago, and the new owner has been driving it since then on the same engine.

 

Glad you got it fixed, will file away the info for future use. Good job and thanks for the follow up.