Back to OS 9 being a bit more “sensitive” and no issues with Tiger…
Are
ALL mobo mounting screws sufficiently tight AND are all those metal surrounding contact surfaces clean and free of tarnish? Just something else to also check / address.
No other cards installed - except for the bare minimum for basic operation?
Think I’d try swapping out the mobo with another B board (or even an A board) just to see if the hiss follows with your CPU & SSD. If not, then it isn’t your OS 9 install… it’s
something with (or on) that B board.
