Check out this video. LOL, the guy is a little funny to watch.
https://www.youtube.com/watch?v=CAY2rRsqHLE
The PSU is not a wall wart one though.
As for the wall wart. If the plug in blades are not polarized try plugging it in 180 degrees from the way you have it plugged in now. See if that helps.
Here is an Agon thread I ran across. This is where I found the above video.
Ground Hum with Rega RP6, Exact and TT PSU.
https://forum.audiogon.com/discussions/ground-hum-with-rega-rp6-exact-and-tt-psu

