I have an amiga 500, and was transferring some files to it when I started hearing a strange noise. It was like a pulsating alarm clock. This noise happens whenever I do any sort of serial communication. Even when I just hit refresh on Amiga Explorer. It lasts the duration of the serial communication. It is output from the amiga via the rca jacks I am fairly certain as it comes into my computer via my line in. I have attached a sound recording. This is the amiga when I am transferring an adf to floppy over the serial port. The serial port seems to function normally.
Note that the static/noise is not from the Amiga but from my dirt-cheap Sabrent capture card:)