I tried running my audio board off a 12v LA battery and it was very quiet, totally fixed the problem. So I left it in there, has not leaked yet.......only kidding.
The 12v supply rail to the audio board is filthy dirty and only just 12v, maybe the 18000uf is knackered.
I also wondered if the wires are running in a noisy environment in the cabinet.
I noticed on the audio board there is a layout for regulation components that are missing and bypassed by the choke.
I think they were going to power it from a higher supply rail and then could not.

I am going to try to fix it properly but it is still not as bad as my 2 system 3 machines what with the turn on THUMP and hiss.

So I am back to this thread 3 years later haha.
Started using Laser War again and it was "normal quiet" at first but then developed a super loud hum.
Long and short of my fault finding so far is -
The L+R audio pre volume control is good.
Left channel audio is good all the way to the speaker.
Right channel audio has hum and some audio, the MB3730 is amplifying what is presented to it.
Center channel is all hum and nothing else, super loud.

So all the hum is generated after the 2 channel audio goes through the volume control but before it hits the audio amps, and is in the right and center channels only. Let's look at the schematic......it does not match. I appear to have the super early DE 0288-1 with the factory ground rework mods. I cannot find a schematic for it anywhere.
I think the later schematic's are functionally correct for my board but the component designations are totally wrong.

Well the right side MB3730 is dead and the center channel opamp appears to be causing the massive hum.
So as I have good audio pre volume control I just added a TDA7297 amplifier module I had kicking around and all is well again.
No center speaker but the sound is quite good from the stereo pair.