I don't remember specifically on which Capitol USA Grand Funk tape it was present (probably WAAB or S'O, not ATG) when i worked the recorded tape for digital conversion that there was a float on very low frequency that had never seen before on other tapes, so i've looked for it and it was a 3Hz noise, constant for all the tape and both programs.
Filtered it out and there you go... but the question on WHY it was present, and it was unique, puzzled me for a while, when i saw on a old 8-track fan page the story and evolution of Ampex 8 track duplicators and by 1973 they were going 20x realtime duplication.
Guess what? a 60Hz hum into the duplicator chain that was running at 20x speed. Back at realtime speed, 3Hz. Talk about quality control...