Ttykuu

I had my first programming experience in the late 80s / early 90s on a Schneider (Amstrad) CPC 464 in Basic.

I remember that when a program was running, depending on the current workload of the processor you could hear a faint buzzing sound from the integrated speakers.

Especially when an empty for loop was running (like for i = 1 to 500 : next) which was regularly used to get short waiting times (as there was no sleep command IIRC), you could hear this buzzing which gradually changed its frequency during the loop.

My question is: Was this a feature (as an audible feedback that the computer was working) or a consequence from insufficient decoupling of circuits?

There seems to be a slight misunderstanding: The CPU in a computer is (almost) never doing nothing (excluding power save states which was largely unknown at times the Amstrad was en vogue). So there's no such thing as "workload of a processor" going up or down - The CPU in an Amstrad is constantly running at 4 MHz, constantly under the same workload. If there's nothing do do, like executing a BASIC program), the CPU is, well, busy idling around at the same speed with the same "workload".

What you possibly can hear, however, is the CPU repeatedly doing the same thing, like operating in tight loops that can, through electrical or mechanical interference (with the monitor or audio signal, for example), create audible frequencies that you may hear. That was definitely not an intended feature.

Operators of mainframes sometimes could hear whether their machines were running properly (or had crashed) by listening to the interference noise they produced, sometimes even tried to amplify the noise. (See this book, page 161ff)

You were experiencing some similar effect.

Today, the sound of historic computers operating can even be marketed as art.