@edlongus I heard the "hum" in the intro and its on my copy of the Greatest Hits LP-the mono/stereo hybrid. I think its more noticeable on the 45 version. Also the vibes in the instrumental break really pop on the mono version.
Wiki states: Some pressings use the single mix from 1970; reissues use a hybrid of the single and the album version from Bridge Over Troubled Water.
That's interesting, I wasn't aware of such an animal!
Track A1 released by Dallas band The Nightcaps in 1961 but was never copywrited. In 1975 ZZ Top took it upon theselves to do just that. [YouTube Video]