The Stovebolt Site Forums The Shop Area Engine Shop Clattering 270

I finally have my '57 GMC assembled and driveable. I installed a 270 I got as part payment for the 400 Pontiac I had in it.
The problem I'm concerned with now is a clatter coming from the engine. This engine shows about 55 lbs oil pressure cold,and 35-40 when warmed up. It sounds okay except when I first start it up,then it sounds like all the rods are knocking,for about 1 second,after which it quiets down. It also makes the same noises when I back out of the throttle,like slowing down for a corner,etc. I can hear the noise even when I'm running the engine stationary,but the clatter is only present when the engine's winding down. Under load or accelerating-no noise.
Seems like if it was rod bearings the oil pressure wouldn't be so good. Anybody have any ideas what's going on with it?
Could it be piston slap or wristpin knock? I used to know all this stuff,but haven't had to fight this beast for a bunch of years and can't remember the symptoms for all these possibilities.
The truck does have some valve problems,but it doesn't sound like the noise I'm hearing is related to burned valves. I have another head getting the valves,guides and seats replaced and the surface being milled now,so it should be ready in a few days;then all I gotta do is figure out a way to bail it out of the shop. I know that'll give it more power and make it run cleaner. Hopefully I can get this knocking cleared up.
Thanks!
Speed

As I read this I immediately thought of piston slap...I can't say why, maybe it's carma?

That crossed my mind too.
Does that occur only when the engine's decellerating? If that's what it is,I can live with it. I don't thrash the engine or anything,so it should be okay for a while. I just don't want any catastrophic failures.
Speed

hope I,m wrong,,but the way you tell it when the noise comes and goes you better pull the pan and check rod clearance...doc

you should be able to get the noise in the driveway and short each cyl out,, if rod noise,,it will leave or change when correct cyl is killed...doc

Ran into this exact thing recently on a friends 228 GMC. It turned out to be the front damper was loose on the crankshaft. It clattered on acceleration and deceleration at certian rpm's. Don't pull the pan yet. We also thought it might be wrist pin or #2 piston. We changed out the damper and put in a new key ~ problem disappeared. Good luck.