I have a 1979 Ford F150, with a 351 windsor. I just had the exhaust ran out the back of my truck, and after that I noticed that I had a slight engine knock coming from the passenger side of the motor. The knock isn't very loud at all, but enough to make you concerned. Also, the knock keeps the same beat all the way up to about 1800 rpms. I've also noticed when held at around 2200rpms the motor sounds like there is a little bit of a rattle kinda knock, and it doesnt keep beat like a normal "knock knock knock". It's more like a rattle, silent, rattle. I'm running 60 pounds of oil pressure and when rev'd it jumps up to about 70. Also I heard that the motor was rebuild when I bought the truck, and has had a beafy cam put in it. I'll also add that the truck runs just fine, and when it warms up to about 160 degrees + the knock goes away completly at idle, but it still has a tiny knock, when a load is put on it.

A little more information.. Its pretty hard to hear the knock on the out side of the truck, but in the cab you can hear it. It almost sounds like a slap slap slap. But i'm thinking that if its piston slap it should go away after at least 5 minutes. And when you first fire the truck up, it doesnt knock untill about 15 seconds of running, then it starts the knocking.

need some help here.
thanks

 

Have you given the ol' header bolts a retorque? Exhaust leaks can give a concerning 'knock'. If that doesn't do it, pull spark plugs (one at time) while the rig is running, if the sound gets better or goes away, you found your problem cylinder.

 

I would drain the oil through a coffee filter or a piece of a white t-shirt and see if you can see brass or metallic.
It would make you feel a little better knowing its not spinning a bearing

 

bought a truck with a 351m top end rebuilt come to find out he didnt touch the bottom. bought the truck off my cousin he had someone else do the motor. it had this same knock/rattle. i assumed it was the rod bearings since he didnt touch it. or possibly the oil pump, never found out it was easier to sell the trucklol

 

I always make sure to wear some heavy gloves when doing this, I've been shocked by plug wires before and it ain't fun. Something i'd try is getting a bottle of Rislone oil treatment. I've had a couple trucks with slight knocks and within a hundred miles you can notice a difference after an oil change and rislone. I'd also check the exhaust bolts as 704 dentside man suggested.

 

Are you sure that since you just had the exhaust done and now you have this knock sound that your pipes are not hitting the frame or cab somewhere?
Try the simple stuff first, get under there while its running of course with the e-brake on let it warm up hell even look while its cold and have someone "craddle" the gas pedal and look around.

 

If you know it is a knock consider having an oil analysis done. Not that expensive and it will tell what kind of metal, if any, is wearing beyond normal. It gives a pretty darn good health picture of the engine.

 

what type of gas are you running? might need to run a higher octane gas, some people have confused pinging for a light knock.