im running a 389 in my 68 tempest, super comp headers, stainless 3" exhaust with delta 40's and a x pipe.
when im cruzing, and let off the accelerator, it will idle down and will emit a low but distinct "boom".
when i smash the throttle, she runs like a bat outta hell.
sometimes when cruzing on the freeway it will make other people look.
i tried to solve this problem by isolating plug wires, changing timing and even rejetting my carb...
nothing worked, it has a pretty radical cam in it ( 290 dur @ 050, with .525 lift 112 centers).
since it didnt hurt peformance, ive learned to live with it.