How do the ROM sockets look? Do the EPROMs fit snug? Do you still get that sound when the EPROMs are removed?

I might have the same problem. My games only get played every few years. My "Time Machine" was working perfectly the last time I played. Months transpired and a few months ago I fired the ole girl up. Humongous hum out of the sound system. I disconnected everything and started at the transformer and worked outwardly. All voltages are acceptable. I gave up at that point and it has been sitting for many months. I do remember that I don't even get to the point where I can watch the LED's on the CPU or any sort of diagnostics. Even when I disconnected the sound board I still have that nasty sound. I read somewhere it might be grounds, PIA chips or the big capacitor in the backbox. Sorry I can't offer any encouragement. I will watch patiently though.

Have you tried the ground float? There is a big long thread here dealing with that. Essentially, it comes down to using nylon washer and/or screws to keep the mounting points off the backbox ground. The connector has its own ground, so it doesn't need to be grounded to the backbox ground, introducing a ground loop. Quick way to find out is just unscrew it and let the board float in the air (not touching anything obviously) and power it up.