Most of the noise is coming from the motor which is firmly attached to a wooden or carbon fiber plank. If you think about it its the same setup as a stereo speaker. Magnets, copper windings and a huge flat surface to make air vibrate.
You could try a thicker riser plate or add a piece of neoprene rubber inbetween the trucks and deck. Adding plastic bushings or rubber grommets to the back side of the motor mount might help too but there probably isnt much wiggle room in your setup without misaligning the belt driveline.
Esc settings wouldnt change much and thats just a small amount of high pitch whine from the 12khz (or whatever setting you use) switching frequency.
Got a pic of your motor and mount?
I am at work now. The link of my motor: Sensored Brushless DC Motor Battle hardened 63100 190KV 5500W This motor is very popular. I’m studying whether the ESC mode is not selected correctly. Unfortunately, I still haven’t solved it. I will try your method tonight.
Another way to look at it is vibrating solids make sound. So what you could do is use a more spongy grip tape or add other sound reducing components anywhere on the board. Kind of like adding Dynamat under the carpets in a Honda Civic to reduce unwanted vibrations and resonance.
I don’t know how much quieter you want it, but switching to hub motors or direct drive would make a huge difference. Obviously a more expensive option. But it’s crazy how much quieter hubs and direct drives are.