Rod knock or valve noise?

[clutchdust]

About 2000 miles ago I overhauled the engine (454) in my truck. #7 piston was blown out from detonation but everything else looked good. I put brand new bearings and rings in the bottom end and figured as long as I had it apart I would install a brand new Comp Cams cam and lifter kit. Got it all back together and did the 20minute/2000rpm break-in thing and all has been good with the engine.
*HOWEVER*
Every so often, mostly when the engine has been sitting for a few hours, on start up I'll get a pretty noticeable clatter that lasts for 1-2 seconds, then goes away. It literally lasts no longer than it takes for my OPG to read pressure. Speaking of, cold oil pressure is just over 30 while hot oil pressure seems to run between 20-25 at idle.
To my ear it sounds kind of high pitched, like valve noise, but I'm a worrier so of course the first thing I think of is a rod knock. But I've never heard of a rod knock that goes away. Then of course, there is the fact that it is brand new lifters so that makes me think something else, even though I know that (rarely) parts can be bad right out of the box.
But like I said, it happens so fast I can't even get out of the truck before it goes away.
Thoughts?

 

[Red77]

What kind of pistons did you use? Forged, cast, hypereutectic? This can make a difference and actually have a knock that sounds like a rid knock that goes away as the pistons heat up and expand at operating temperature. Not saying that's what it is, but it is common depending on how your piston to bore clearance is set up and what pistons you've got installed. Some manufacturers even offset the piston pin bore in the piston slightly off center to help combat this noise issue. My 1992 F-150 made the noise for the first 2-5 minutes of operation for the 10 years I had it. Sounds very much like a rod knock. Some people even call it piston rattle. We warrantied a few 5.0L Explorer and Mountaineer engines back when I turned wrenches because the customer couldn't stand the noise.

 

[SuperBuickGuy]

Check the lifters to see if they've (or one has) collapsed. A guy I know has had that problem with his BBC as well

 

[DeeVeeEight]

Check the lifters to see if they've (or one has) collapsed. A guy I know has had that problem with his BBC as well

I agree. How thick is your oil? I had a similar problem. My break in oil after rebuild was too thick. Switching over to a 10w 30 helped a lot. I also like to spin the motor for a few seconds before I engage the choke, to let oil pressure build before the motor races on start up.

 

[clutchdust]

What kind of pistons did you use? Forged, cast, hypereutectic? This can make a difference and actually have a knock that sounds like a rid knock that goes away as the pistons heat up and expand at operating temperature. Not saying that's what it is, but it is common depending on how your piston to bore clearance is set up and what pistons you've got installed. Some manufacturers even offset the piston pin bore in the piston slightly off center to help combat this noise issue. My 1992 F-150 made the noise for the first 2-5 minutes of operation for the 10 years I had it. Sounds very much like a rod knock. Some people even call it piston rattle. We warrantied a few 5.0L Explorer and Mountaineer engines back when I turned wrenches because the customer couldn't stand the noise.

This was just a bare-bones overhaul. Factory pistons came out, factory pistons went back in. The only things new inside the engine were the cam, lifters, rings and bearings.

Check the lifters to see if they've (or one has) collapsed. A guy I know has had that problem with his BBC as well

I agree. How thick is your oil? I had a similar problem. My break in oil after rebuild was too thick. Switching over to a 10w 30 helped a lot. I also like to spin the motor for a few seconds before I engage the choke, to let oil pressure build before the motor races on start up.

I'm hoping it's just a noisy lifter, but like I said, the worrier in me likes to think the brand new lifters are fine and that something else is the problem. Only good thing is most of the time when I worry, all it costs me is years off my life.

I'm running 10-40. OUt here in "holy hell it's hot" land, that's the thinnest oil I'll run. Actually thought about switching it over to 20-50.