Hicap noisy

I’m saying noisy as it’s not a hum or a buzz, it sounds like a fish tank pump running.
It comes and goes with no rhyme or reason and varies in how loud it decides to be.
I’ve been tolerating for a while.

Now here’s weird bit. It only does it when it’s very near the glass shelf of the rack.
Nothing to do with touching the shelf, as you can hold it about a 1/4" from the shelf and it still does it.
The noise reduces to an acceptable level, in stages, as you get to about 3" from the shelf.

It’s almost as if the shelf is causing a reflection.

It’s a temporary rack. It’s a wide, 3 shelf AV thing, so I even had a go at moving it to the other side, just incase it was something to do with the NDX2 underneath, however it does exactly the same at the other side.

Most odd.

What happens if you stand it on its side? I wonder if it’s a loose toroidal.

Still haven’t had a chance to investigate further but I’ll give that a try and see what happens.
When I discovered the thing about it’s proximity to the glass shelf, I kept the Hicap fairly level.