> Why do I associate the noise with Acorn computers?
Perhaps because when the CPU is idling it's executing a few instructions (to check mouse movement/clicks etc) over and over... with RISC OS being so much simpler than other OSes, and being written in assembler, you have a shorter loop (hence higher frequencies) and it has a more obvious periodicity (more tonal rather than white noise).
I've heard of engineers on older computers using this sort of side-effect to debug code - really! In fact I've done something similar myself a couple of times.
You're not a true geek until you've debugged your software using an oscilloscope and/or logic analyzer!