I remember using that stuff w/ my Dad on his '71 Polara....best do it away from home or someone may call the fireguys? .
It does sound oil related....have you run a few oil changes through it? . Sometimes a few changes of different viscosity works and one will 'take'? .
I've found exhaust obstructions before that would touch the system at certain revs only even internals of mufflers loose and sounds traveling up the pipe to the front. .
Make sure the trans has no loose bolts also that the flexplate is Ok if an auto. . Same w/ trans mount and crossmember bolts. .
And have fun! .
~ Pete