About 15 years ago I freshened a set of 516’s a guy had been running on a 440 with the MP620 cam.
I never saw the cam or lifters, but he said they looked fine, and he continued to run them after he got the heads back.
I have no idea what springs they were, but they were set up at about 500lbs open...... and I set them back up like that.( Btw, using the specs from the load chart in the MP engine manual, the recommended spring for that cam shows 491lbs open load @.600 lift.)

Around that same time frame I was doing a fair about of dyno work for some guys that were building dirt track SBC motors.
They were using fast rate flat tappet cams, and the first couple I ran for them had some pretty serious valve float issues.
The cam supplier had told them to use their springs and set them up at around 130/330....... which just wasn’t getting it done.

The motors we built for that class were using springs set up more like 150/370, and they would rev about 1000rpm higher.

As to the OP’s strange noise....... IMO, it’s related to the cam bearing clearance and or material.

This is the test I would do......
Remove the spark plugs so the engine can be turned over manually as easy as possible.
Verify you’re still hearing the noise.
Then remove the distributor and drive gear...... install and run the priming tool, and see if the noise is decreased or eliminated(or if the motor seems to rotate easier) when there is oil pressure at the cam bearings.

I’d be trying to figure out if the noise was related to the cam rotating in the bearings, or the lifters rubbing on the cam.

Another thing that could be done is remove most of the pushrods.
Say, all but one cylinders worth.
This would greatly reduce the friction between the cam and the bearings, but the two remaining lifters would still be rubbing on the cam just as hard.