It may not even be related to the Mic circuit - but an open line noise from a wire that's fallen off a switch on the front panel. Like NB ANL for the receiver side or like the Roger Beep - can inject a noise loud enough or make the ground plane around the 4558 chip noisy enough to pick up the hum. Especially if it's sounding like a power supply hum.
So Volume, Squelch, NB/ANL - any audio driven line can inject the noise.
Possible that wire or ground plane loop has showed up - usually from the radio taking a tumble off the table or got banged around more than it should - Galaxy 44's are older radio so just about anything even a loose Mic Plug Jack ring can cause the noise even with different mic's.
So for your efforts, take the panels off the radio - especially the component side - speaker. Look around and as you can move the loom wires harness as well as the individual connector headers - looking for and listening for changes to that hum.
Even the front panel mounting as well as side jack - both are injection points - if the front panels bezel as well as the control PCB's themselves - any loosening or recent work can cause this too - just have to reseat the panel PCB's - tighten controls' hardware and side panel screws can help.
As you work your way thru, you may find it in the headers - to the front panel or to the main PCB. These areas are known for open traces from lines that go somewhere but are from features that are not used - just empty traces - these can eventually get coated with dust fuzz and solder lint that they may fail to ground or to a working circuit and act like antennas. Even to the back panel...
If you can't find it in the harness, try the bottom trace side and run your fingers along the traces from the headers back to the Audio Chip, again listening for changes to the hum - it may also indicate you have a filter cap problem and the chip is more prone to noise pickup caused by the cap failure in progress.
It's one of the hardest ones to solve yet once you find the open line or failed part - you slam yourself for not thinking if it sooner...
I have The T-shirt with the Trophy - and I occasionally use the Sweatshirt to dust the cabinet.
:+> Andy <+:
