340 -727 -3000 stall- Vibration under load - Trying to find source

I’ve searched many threads on vibration etc before posting here.

Engine/trans combo info:
Steel crank internal balance early 72’ 340, balancer rubber in good shape and not slipped.
727 rebuilt with 3000 stall converter. Converter foot brakes to 3000 maybe even 3200 before it boils the tires off. Don’t know brand of converter, was told it had bearings from the seller of the car. I checked, No weights on the converter. Transgo shift kit. Shifts super crisp.

trani mount okay. New rubber motor mounts. Took fan belts off to eliminate fan or alter/water pump as source etc…. Checked for converter weights - none found.

the problem:
Have a droning vibration predominant at off idle to 3000/3200 rpm ish then smooths out when above that range (still very very slight above 3200). Vibration is in any gear under load and at constant throttle settings.

When rolling/driving in vibration rpm range (1000 - 3000) and click into neutral and engine idles down but car is still travelling fast the vibration essentially goes away. Tells me not tires, or driveshaft/u-joints etc. on pulls above 3200 ish iP to 6000 rpm fairly smooth, liveable anyway but you can still get ever so slight Light drone sensation.

when I say Droning vibration I mean the vibration is pulse like, comes in and out in a constant repeatable rhythm.

I’m thinking it’s the converter poorly made or not properly balanced. Want this gone… any help appreciated

Thoughts?