Caution: deep rabbit hole.
BBC Rewind has a collection of freely downloadable sound effects available, as well as a mixer mode you can use to play around with them to your heart's content. For the deep-cut retrotech enthusiasts among us, I call your attention to the Electronics section, but there's a lot here to enjoy.
https://sound-effects.bbcrewind.co.uk
and
https://sound-effects.bbcrewind.co.uk/search
and
/nosanitize
https://sound-effects.bbcrewind.co.uk/search?cat=Electronics
.
@corlin Here have some more!
The Gold and Red Libraries (Gold effects start with “G”, Red with “R”)) consist of high-quality, first generation copies of original nitrate optical sound effects from the 1930s & ’40s created for Hollywood studios.
The Sunset Editorial (SSE) Library includes classic effects from the 1930s into the ’80s.
https://archive.org/details/usc-sound-effect-archive?