My cast 4" eagle 360 style crank is balanced internally. My 318 stuff swapped right over. No weights bolted on the aftermarket damper and neutral balance flywheel. I did have the crank balanced to match the pistons n rods etc

My guess is you have something hitting in a tight spot like exhaust maybe, transferring vibration. Motor mounts are rubber? My car is no crazy beast but a low idle can have a serious drone. If there is a sympathetic vibration at a certain frequency that will always hum at that rpm...?

Good luck