I have this exact issue. I only notice it when listening to the 3.5mm speaker jack through studio monitor headphones. Unsure if the IIsi ever had "clean" sounding rear audio to begin with, as I postdate the computer, myself, and have never used one when they were new. Very clean PCB, no corrosion, aggressively cleaned/neutralized and thoroughly dried, recapped with tantalum capacitors. Has never had chimes of death or anything weird unless I've purposely crashed it. Has been running fine for years at this point.
It's just a faint buzz when moving the mouse, so I'd second the ADB noise theory. Still stumped as to whether this is "normal," or at least nominal. If it isn't something nasty that's happened to the logic board, I am personally fine with it. Have heard far worse on some old PC motherboards before, without it being anything physically threatening to the PCB itself. ZuluSCSI is powered with a Molex->Berg adapter for power, I did not bother bodging termination power. My power supply is a PicoPSU with a pretty high-quality/amperage DC brick.
It's just a faint buzz when moving the mouse, so I'd second the ADB noise theory. Still stumped as to whether this is "normal," or at least nominal. If it isn't something nasty that's happened to the logic board, I am personally fine with it. Have heard far worse on some old PC motherboards before, without it being anything physically threatening to the PCB itself. ZuluSCSI is powered with a Molex->Berg adapter for power, I did not bother bodging termination power. My power supply is a PicoPSU with a pretty high-quality/amperage DC brick.
