[Floats]

Mech 3,
I can’t help, but I believe I have a similar problem. I think it is the crankshaft moving forward when the inertia of the car is driving the engine. Ie gearing down and using the engine to slow down. I believe the drive train was not designed for this ‘abuse’
The crank’s fore and aft movement specs are just a few thou.
Furthermore, I believe poor restoration/babbeting and modern driving techniques( gearing down in order to slow down) does not bode well on the crank thrust washers/supports.
I could be mistaken and will let you know if and when I have had the opportunity to drop my oil pan.
Honestly, I think that is the noise you are referring to.