Infernal Knock!

[John_Aldrich]

Knock in the engine. At first it sounded like the center main. Removed one shim on each side. Then it sounded like number r cylinder. Pulled one shim each side.
Now it sou vs like number one cylinder.

Ideas as to why it seems to be migrating?

[TXGOAT2]

Maybe the crankshaft is slightly sprung. (Or developing a crack) Another maybe: Something loose in flywheel assembly. Or maybe it's just a bit of carbon hitting the head. Or maybe the oil pipe has broken loose or plugged up. Cam bearing?

[Norman Kling]

Unfortunately, the center main is usually worn by the 4th main sagging which lowers the back of the transmission and raises the center main this causes wear in the block and not the cap. So tightening up the cap by removing shims will only cause the crank to continually flex in the center eventually leading to a broken crankshaft. So the best fix would be to remove the engine and pour all the main bearings line bore and also straighten the crankshaft and straighten the crankcase. Which is a lot of work, but in the long haul will save your engine.
Norm

[Dan Haynes]

It might be a twisted rod. If the wrist pin and crank journal are not parallel. It's most often caused by holding the rod in a vise while tightening the wrist pin clamp bolt.

If the two are not parallel, as the crank rotates, the rod will torque and the big end will slide to the end of the journal, knocking on it when it gets there. Annoying because it will knock even with perfect bearing clearance.

[John_Aldrich]

Unfortunately, the center main is usually worn by the 4th main sagging which lowers the back of the transmission and raises the center main this causes wear in the block and not the cap. So tightening up the cap by removing shims will only cause the crank to continually flex in the center eventually leading to a broken crankshaft. So the best fix would be to remove the engine and pour all the main bearings line bore and also straighten the crankshaft and straighten the crankcase. Which is a lot of work, but in the long haul will save your engine.
Norm

This engine is fairly new but when rebuilt the babbit was not re-poured because it looked so good. Rebuilt by a reputable T mechanic (not me). I'd really hate to pull the engine and have the entire thing rebuilt again.

[TXGOAT2]

Check 4th main condition (?)

[Scott_Conger]

There is only one person that you should be talking to: the guy that did the engine

not that it is in warrantee, but because he knows the engine and not another single person on earth does.

removing shims hither and yon does not appear to be sound diagnostics since you're still at a loss. I am at a loss as to how you can remove those shims and get away with it on an engine that is refurbished but that's another topic for another day.

a bunch of people on the internet, offering their opinions with less information than you personally have (seeing, smelling, hearing) is a recipe for ...well it all depends on how many wrong things get fiddled with before the actual cause is discovered.

I feel for you in your frustration, but this approach is going to lead to a lot more of that. In spades.

[TXGOAT2]

When the engine was rebuilt, was any attention given to the transmission?

[Wayne Sheldon]

One I had many years ago. And very easy to miss unless you take it all apart. A flaw in the upper end of the connecting rod made the clamp onto the wrist pin, shall we say, unreliable? That engine I had work done by someone else, that in turn farmed some of the work out to another person.
All of a sudden one day, I had a nasty and very loud knock. I was in overdrive, doing about 70 mph in my speedster, and idled the engine down, expecting I had thrown a rod bearing. Coasting onto the nearest exit in neutral (using the hand brake) I listened to the idling knock, and it went away. I stopped on the shoulder of the exit, raced the engine, listened intently, no knock. Got back onto the freeway, and gently increased my speed again. About ten miles later, the knock started hammering again! Again I coasted onto the shoulder of the freeway (the nearest exit was too far away). Again. the knock mysteriously disappeared. Convinced now that it was something odd, and not an immediate threat to blowing the motor, back onto the freeway, and I completed the days plans with the knock only shouting out loud for a few moments here and there.
Over the next few days, I spent considerable time examining bearings, timing gears, transmission, and listening with listening devices when the knock would show up and speak out. I didn't find anything definitive. Basically, it was near the front of the motor, and nothing appeared to be in any danger of serious imminent disaster.
I continued to drive the car, and often hard, once I got used the knock's mysterious coming and going. But it continued to drive me nuts. I even drove it on an Endurance Run! But I didn't like it. So, after another year or so, basically I had faith the car wasn't planning to let me down? But still, I didn't like that knock. I was planning to attend a national club tour, so I decided to figure out once and for all what it was.
So just as Scott C is suggesting, I pulled the engine out, and tore it almost completely apart.
I had previously concluded the trouble was most likely a wrist pin, either number one or number two. I had as best as one can checked the wrist pin bolts from under the engine, and determined nothing. But the teardown confirmed it! Number one wrist pin had nasty looking and very not right wear on both sides of the notch in the wrist pin from the wrist pin working loose and turning back and forth knocking against the wrist pin bolt. It clearly would wedge itself, stopping the knock! And later again become loose to knock like crazy until it would wedge itself again!
In retrospect, the wrist pin was probably a bit too tight in the piston. However, the real culprit was found upon careful examination of the upper rod end. A minor flaw, a slight ding in the rod itself caused the clamp to at full over-torque tight, be only pressing onto the pin on a couple very small points. Wedged just right, it would hold. Slip off just a tiny amount, and the pin would 'float' within the restricted space of the wrist-pin-bolt notch. As it spun back and forth, it would knock the notch and the bolt together.
About ten seconds with a sharp rat-tail file took care of the ding distortion. I carefully assembled the rest of the engine, and very much enjoyed the national tour, and several more years of knock-free touring and Endurance Runs!

One of the most common harmless knocks is slightly (even VERY slightly!) worn steel timing gears. The size and timing of the cam lobes cause pressure on the cam shaft to jump forward and backward very slightly several times per engine revolution. If those steel gears have even .001 excess play, they will knock. However, that knock is not loud, but can be heard, and drives a lot of people crazy. Since I have always worked on a tight budget, I usually have had original timing gears, and have gotten used to the peculiar sound of their knock.

However, speculation by all of us out here in the hinterlands is just that. Speculation. Unless you have good reason to trust that it isn't leading to a sudden and massive failure (I sometimes look back at my year of driving with that stupid knock as being young and foolish?)? I suspect Scott C is right (he usually is!), and you may need to just bite the bullet and tear it down to see if you can find it.
Carefully check the fit of everything. Pay special close attention to the line bore of the main bearings, cam bearings and gears, and don't forget the wrist pins!