i have thrown everyone a curve on this one maybe some of you guys can help.i have a 70 f250 with a 400,after a little work i have it running beautifully only problem is when i put it in gear it will knock,take it out of gear it will stop,the thing that is puzzling is that after it runs long enough to warm up good,it will slightly knock but when you accelerate it stops but let off and it will start it up again,but it is not coming from the top end and it is not coming from the block!help i need to figure it out quick!any ideas and suggestions would be very helpful.thanks everyone for all the help on previous times.
thx
steve

 

Sounds like you have the identical problem I have on newly rebuilt 390. Did not have this problem before the rebuild. Since then, I have a noise that is most noticeable upon start up, and the loudest with the truck in gear, and when the RPM icreases just above idle with a load on it. The noise deminishes as the truck warms up, and almost disappears after 10-15 minutes of driving. Since getting it going after the rebuild I have added an edelbrock HP intake manifold, Holley TBI fuel injection sytsem, Mallory unilite distributor and ignition sytem, 427 type adjustable rocker arm assemblies. I have replaced every vacuum related piece on the truck to include power brake system, transmission shifting solenoid, etc. The noise persists. The only thing that seems to have helped at all was installing the adjustable rocker arm assemblies. It may be my imagination, but it doesn't seem quite as loud, and dissappears quicker since that modification, but the noise is still there.

I wish I could find the source. I am sure that it is NOT: pre-ignition, Main or rod bearing, wrist pin, or tappet noise. I have done enough of this kind of stuff to recognize all of these.

One shop I went to for analysis said that they suspect that it was "piston rattle" caused by the new jugs being just slightly undersize to the new bore. They claimed that if this were the cause, it was nothing to worry about if it disappears after complete warm up. They claim it is pretty common in rebuilt big block engines.

I will keep chasing this, and let you know if I ever solve the mystery. Please do the same.

Is your engine just recently rebuilt?

 

What if your flywheel{flexplate is more like it on an automatic} is cracked,has loose crank bolts,or loose convertor nuts.Might even be just a bad convertor itself! Look in this area before too long.
Phillip