I recently bought an old Macbook Pro (A1398).
The internal speakers make a buzzing sound; anything plugged into the headphone jack also does.
Is there any way to determine if it is a software problem or a hardware problem?
(I know the hardware explanation seems unlikely, but only the buzz in the internal speakers varies with the volume, so the cause may in principle be different).
Hopefully the answer can be of interest for anyone who wants to learn how to tell a hardware problem from a software problem in this kind of situation!