You’ve checked to make sure all the ground connections are good? (Including the screws that hold the boards to the head.)

I have a Taxi that can sometimes have a pretty loud hum when I first turn it on but it goes away pretty quickly I’m not certain what’s causing my issues but it sounds like probably the capacitors are worn out on mine

I just put a Kohout 11C power supply in my Earthshaker and noticed a hum in audio too...after installing it on my Earthshaker... so I'm following. My old PS was bad and MPU was having issues so I can't test it either.

If you still have the original you should try rebuilding it. It’s about $20 in parts for all 7 capacitors and it works great for another 25+ years

Ok in decided to Hook Up my old power supply and not the 100v which had the issue: I can say that the was a audio hum with old power supply too! So I don’t think it’s the power supply with my Earthshaker. Ok 3:48am now I can sleep! LOL I was sitting in bed wondering if I had the hum before the new power supply. Apparently I did (it’s a new to me project pin)

I plan to rebuild my original ps as a backup but I like the Kahout upgrades and build in volt meter

If it’s not the power supply causing it try using Nylon screws on the MPU and Sound board to help reduce the ground loop and reduce noise. Alternatively you can pull all of the boards. Sand the areas where the screws go and clean the metal parts up and that could also help

Not sure if this will fix your problem, but ferrite beads installed in the right areas may help. Here's a thread covering the topic:

https://pinside.com/pinball/forum/topic/jokerz-hum-solved#post-5780250