Strange drive train noise 🤷

Needs some ideas here folks! Just finished (I thought) my ‘64 Cj3b restoration. All stock except the 225 V6. All new clutch parts and most everything else. Here’s the prob:
-after starting and driving all is fine: shifting, clutch engagement etc
-after about five minutes and only after the engine warms up a strong fairly loud metallic? vibration begins - not during acceleration, only when letting off the gas.
-there is also a rattle, like something bouncing around in a can.

I’ve pulled the v6 flywheel inspection cover and can’t find any bolts or anything in the bell housing.

Any ideas?? My bro says it might be the tranny input bearing… but only when hot???

Looking forward to ya’lls input😊

Right off the bat my first thought is the exhaust heat riser in the exhaust manifold, depending on what you have on the engine conversion. Take a look and make sure the coil isn't getting weak and allowing the valve to flutter when it warms up. Since you aren't noticing engine miss fire, I would think the motor is OK. It is hard to explain noise in writing. Give that heat riser a look if you have one. It should be tightly closed on a stone cold engine and tightly open when it its warm (don't burn your hand). If that isn't it we will keep guessing (looking)!

Bmorgil may be on to something, or at least it is a starting point in the realm of possibilities. I had a '79 Chevy heavy half in college. One day I fired it up in the morning, and it sounded like a no question rod knock. It ended up being the heat riser spring broke, and the damper was flopping around at idle. And I was so set on dropping in a big block, but no sense in pulling out a good engine, when you don't have to.

Hahaha! I had to laugh at the rod knocking gm. You bet it will scare the heck out of you if it gets loud enough. Like a fully collapsed lifter, you would swear the rods are coming out the bottom. You start to think about when how and where you are going to pull that motor.