I just updated to 1.23 and short before that in 1.15 I noticed that the sound that get's generated from something runs on if you quit and return to the main menu. This is still the case with 1.23.
The list what 'something' could be is the following: geogen, pump, miner (not running), mfsu, batbox, lv and mv transformers.
I haven't pinned down which of these does that sound, it's a more or less gentle humming with a 'tap' or 'knock' in it's loop but if I'd have to guess I'd blame the gen.
Slight offtopic: is the list of known bugs not going to get updated anymore?