Finding the source of a new hum?

[Cutshaw]

I recently dismantled and rebuilt my pc(I had to fit a cpu bracket and the motherboard doesn't have a cut-out) and now I have a rather loud hum.
I've got a xonar essence st, and it's always been in the bottom slot as far away from the gpu as possible. I don't use the front panel either.

I'm going to tackle the problem tonight, but I've never had to deal with this before and I'm wondering what kind of things I should look out for, or things I should try.

What I really don't want to hear is that the bracket between the motherboard and the tray could be creating some sort of earthing issue. If that is very likely, break it to me gently!

 

[Cutshaw]

e'gads! I've been in denial since it started. But the non-conductive insulation doesn't sound like a bad place to start. What a grim job, all my beautifully pointless cabling was for nought. I'll see how it goes.

 

[Cutshaw]

Well, a bit of an update on this for anyone that cares. I eventually found the gremlin creating all the racket. It's my new(ish) monitor. The 3.5mm jack in the back of the monitor is loose. A gentle push up on the connection and the hum coming from the amp is gone. For various reasons, one surely being stupidity, I never thought to check for a fault in the monitor.

If I didn't use the HDMI for the xbox, I probably wouldn't care, but I've sent a ticket to Benq and I'll see what they have to say.

Cleverclogs here had a summer clearout and flogged or gave away every monitor but this one, hurrah!