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This just happened very suddenly while accelating uphill in 2nd gear. Very loud gear grinding noise but was not shifting and tranny stayed in gear. Sound continued after shifting into 3dr gear. Got car into garage sounded like it was coming from valve side of engine. Ran rpm's up to 1500 or so and then soud went completely away! Idled at 1200 rpm's for 15 minutes, no problem. Drove up driveway and it happened again, put in clutch reved engine and sound went away. This is a mystery to me.
Thanks

I guess it to be something in your drive train.

(o{}o)

Maybe a silly idea but both times when you said it happened you were going uphill but then it settled down on level ground... Since you said it seemed noisy on the valve side I'd think something had happened to oil pickup when the oil all went to the end of pan on the incline and it was lifter noise.

When you made it go away the last time by depressing the clutch were you still on the hill when it went away? If so, and it was just a clutch motion then maybe not oil so how are all the springs on the clutch linkage. Anything free to move or rattle when in the relaxed state and weight has shifted. How about motor mounts?

There is not much on the valve side gearwise to rattle -- actually not much of anything over there unless something happened to one of the exhaust supports and a portion of the exhaust is setting up a vibration.

Maybe a damper spring flew out of the clutch.

Although this sound is straight forward, having hard time clearly describing. Tet me try again. The first time this happened, drove into the garage kept the engine running, the sound exactely tracked engine rpm's. Ran rpm's up and down after about 5 minutes the noise stopped. Continued to modulate throttle no more noise. To me this is behaving like a stuck lifter, but the sounds more like I missed second gear, it's that loud. This is a 1940 356 engine with hydraulic lifters not sure what a stuck lifter on this engine would sound like.

While it is making the noise you might be able to use a stethoscope to help locate the area of the noise. I lew of a stethoscope, I've used a long piece of metal bar by holding one end to the area of interest and the other end up to my ear.

Doesn't sound like a sticking hydraulic lifter to me, though your description doesn't trigger any suspects. You might consider making an audio file of the sound and posting it here so we can listen.

Any chance the fan or water pump pulley is loose and the fan is hitting somewhere? Or a generator bearing beginning to fail? You might just remove the fan belt and evaluate.

Thank you everyone for you thoughts. Noise has not come back. So I willl just continue to dive and if it returnes will look in the areas suggested.
Thanks

Bearing in the generator or water pump? Either can come and go.
Wheel bearing?