I have a 5.4L 99 2WD F-150. When I start it up, there is a slight knocking sound. Within a few seconds it goes away. Sometimes when the truck has been sitting for a while, it will continue to knock for a few seconds after initial acceleration. It always stops with in a few seconds, usually no longer than 45 seconds. You can not here the sound unless you are outside of the truck. You can here it inside just as long as the A/C and the radio are off, but it is very faint.

What do you think this could be. I was reading the boards last night for the first time and ran across a couple of mentions of piston slap, but nothing really specifically describing it. If you guys could give me your opinion I would greatly appreciate it. I also had the engine rebuilt last summer, do to a minor gasket leak. The mechanic doing the work, said that the engine looked great. What else could cause knocks like this?

Thanks!

Also, there are 129,000 miles on it!

 

Went to town this afternoon and there was no knocking. Kind of weird.

 

Mine does it to. 1998 F150 5.4 But only when the temp. gets down below 5C (40F). I just let it warm up for a minute and its usually gone. When its really cold like -30 if I don't warm it up for a couple of minutes it sounds like a diesel engine. I've got about 70,000 miles on mine. Apparently it does not do any damage to the engine.

Rick

 

Mine does it when the engine is cold while under acceleration. Once the temp gauge comes up into the normal range, it goes away.
It most likely is piston slap.
Octane

 

I appreciate the replies! I am one that tends to worry a bit to much, so this has been driving me up the wall! This is very inconsistant and stays at one tempo it never speeds up or slows down. And again, I don't hear it all of the time.

Thanks again!

 

My 4.6 does it on initial startup for about 20 seconds. I just always attributed it to high mileage.

 

You might want to check in the Tsb's section of this site , I remember their

being some bulletins on cold engine knock/tick.

 

Mine has been doing it for the last several years. Same symtoms, a little tick, would go away after a minute or so after start up. Now it got very loud and can be heard under acceleration inside the truck (More of a knock now ). Didn't realy worry about it til now. I parked it and crawled under the truck the other night and had an assistant rev it a little. The noise seemed to get louder and faster as the rpm's went up, sounded like a rod knock. This morning I started it up, the knock was there, but it didn't seem to increase with the rpm's going up, infact it seemed to quite a little. Not really sure what I have here. By the way, the motor is still very strong. Any ideas would be appreciated. Rich.

 

Octane mentioned "piston slap". Almost all of the engine manufacturers make their engines with a wrist pin offset. This means the wrist pin is not exactly in the center of the piston...rather it is offset to one side 0.000-0.100". This is done to combat piston slap by allowing the piston to get ****-eyed in the cylinder. When your engine accumulates miles, the cylinders will eventually wear and [exaggerated] look like a barrel. This will increase the clearance between your cylinder wall and the piston, which will also decrease in diameter through general wear. When your aluminum piston is cold during start-up, your clearance is the largest it can be and at this point the "slap" will become audible. As the piston warms up, it will expand and reduce the clearance back to a point where you can no longer hear the slap.