Have you investigated the amp, connectors etc.
What you describe sounds like ground loss somewhere.
A little - the noise is present whether or not the turntable motor is plugged into the wall. The noise is also present regardless of which Phono input it is connected to (McIntosh C24 has 2 phono inputs). My Luxman and AR-XA do not produce the sound when they are plugged into either Phono input. And the noise gets louder with more volume, and also creates a very loud thump when the C24's input switch is turned to whichever Phono input it's connected to. The other turntables do not produce such a thump. If I remember correctly that's all the testing I've done thus far, seems to me like that is good enough to isolate the issue to the XD's tone arm wiring, but any input is appreciated ....
I may test continuity in the ground wire tonight, and perhaps re-strip the ground wire and solder the spade to it tonight, to be sure it's a good connection.