Hot Rods Engine sound... knock knock, who's there?

The scenario: '64 327 SBC. Rebuilt years ago by my dad in '71. Can't have many miles. Hasn't ever been a daily driver. He's not around anymore to ask. Was parked years ago and unearthed from his barn recently. Given his attention to detail with other areas of the vehicle, don't have a lot of faith in his attention to detail. That not picking on him, just what I've noticed and objective fact.

After fixing the rusted old exhaust, I could finally hear the engine itself over the racket. Has a suttle knocking sound that is at valvetrain speed. Doesn't sound like the "tap" of a lifter. Oil pressure is 75 psi at idle, has a high volume oil pump with 10-30 oil. With revving engine it doesn't get louder or quieter or change frequency. Valves are recently lashed, but I did it with engine off. Noise gets more suttle as the engine gets to operating temperature. Happens with fan belt on or taken off. Is quiet enough that with hood shut is unnoticed. Clutch pushed in or not, is still there. I know it had a steel crank, I saw it years ago when I replaced the oil pump and pan gasket. It had a leak and he wanted me to put in a high volume unit at the same time.

Not too worried about it. It'll get louder and worsen or it won't. Suppose I could pull the pan again and take a look, adequate room under there to do that.

Any idea?

 

Wrist pin........
Wouldn't worry about it.
Valve train speed?

 

Yep, as opposed to crankshaft speed.

 

Cracked skirt on a piston? Pull one wire at a time to see if it goes away.

 

Yeah, that's on my list, good call. I was also thinking about checking the fuel pump. I've heard they can slap if the spring gets weak, but that wouldn't explain how it gets quieter with the motor warming.

 

Forged piston slap?

 

That was my first thoughts

 

My Nomad had similar symptoms in 2000. It knocked at start-up, got quieter as engine warmed up. In my case, it turned out to be #6 piston wrist pin.

 

Its a long shot but I've seen two small blocks with broken cranks knocking like a bad rod. Broke between the chain sprocket and the front journal. Jagged break , still ran decent but didn't sound good at all.

 

Thank so for the posts, y'all. It likely doesn't have forged pistons, it was rebuilt with the stock parts from what I gathered from our last conversation about it. The steel crack was stock, fairly common in that era I'm told. Engine was out of a moving truck.
That said, he likely didn't redo the wrist pins when he did the job. I plan on doing the spark plug wire pull test and see what happens on Monday. On a family weekend now. Stay tuned!

 

I started it up today, got it nice and warm, and started pulling spark plug wires all the way around. No changes in the sound. And since I ran it for an extended time, it got even quieter to the point where you almost couldn't hear it. It does have an uneven "crank" when I fire it up though, like one cylinder has less compression. Going to do a compression check next. He told me he had the heads redone professionally way back when, but that's when leaded fuels were common. As it is, I'm running lead additive right now in the gas. Still has the original Power Pack heads on it per the numbers and the triangle casting on the front of the heads. Wondering if a valve is making noise, hmmm....