I have a 1979 Oldsmobile Cutlass Supreme w/70,000 actual miles, a 4.3L engine, and an automatic transmission…recently when placing the car in reverse (after a cold start) I hear a “ticking” noise…not loud enough to be desribed as a “knock”, but “ticking” none the less…Once the car is warmed, for instance after my 7 mile drive to work, I can park the car, then place it in reverse with no noise…Also, the car exhibits no other mechanical abnormalities, i.e. hesitation, slow shifting or slipping
Your post tells me the ‘ticking’ noise is only there when the engine is cold.
That leads me to think you may have a dirty or sticking lifter.