My truck is a 1979 F250 460 4speed 4wd. A fairly loud knocking sound comes from the flywheel/clutch area at higher rpms. The knocking stops temporarily when accelerating or decelerating, but then starts back up with a steady rpm. The sound is present in every gear, so I ruled out the transmission. What could it be?

 

OUCH! Last time I heard sounds like that, they were followed by the loud screeching of a spun bearing(wasn't a Ford, so I didn't fix it). Sorry-not tryin' to bring you down but it does sound like a serious lower end problem.

 

Also could be just a flexplate with loose mounting bolts. They can make a helluva racket when they are not torqued down properly

 

flexplate is easy enough to check to rule it out......

 

I don't think 4 speeds have flexplates, most of ones I've seen have a flywheel & clutch instead.

Before you do anything drastic, check to be sure your harmonic balancer on the other end of crank isn't spitting out its' rubber insulator strip. Sound will telegraph through crank & bellhousing can act like a guitar body.

Also make sure your flywheel is not loosening up. If it's loose, it's detectable without pulling more than inspection plate. Retorquing it however is a different story. Also if it is flywheel loosening, be sure to use NEW bolts when you retorque it!
FBp

 

oh-sorry, I didn't even notice we were talkin' about a manual tranny.....

 

How long have you had this truck? How long has the motor been in there? Does the motor shake too?

It could be the wrong flywheel. If the motor is balanced externally and it has a zero balance flywheel or the other way around it could be your bearings getting hammered by the out of balance crank. Something to look into if this is a new set up.

This is not a factory set up, so really it could be a lot of different things.

 

I have had the truck for a little over a year. The rebuilt motor is out of a 76 Lincoln and was in it when I bought it. It runs as smooth as can be. This is an internally balanced motor, and since there is no "correct" flywheel for this setup, a counter balanced flywheel was the best that we could find.

 

Whoa, wait a minute, you don't have a zero balanced flywheel on an internal balanced motor?!?! You need to change to a zero balanced flywheel. You have a couple of options. You could take a 360/390 truck flywheel and drill the 460 bolt pattern into it and use that or you could get a new flywheel. L&L sells them. You can do a search on the matter, it has been coverd before.