I had an otherwise good circa 1979 green Mustang that always made this 'koonk' 'koonk' 'koonk' sound when revving at the limit and under load.
I have the same thing with one of my power torques as well. My theory: the quality of fuel messes the detonation timing and is like a car pinging.
My yellow Vortex makes that sound at low revs and sounds fine when revving!

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!