First, no, it's not my transmission. Although I do have a separate whining noise coming from there...

All of this is while stopped:
I hear a very high pitched, constant, noise while I'm in my car. I would call it similar to having my ears ring, but a bit quieter. The odd part is, when I press the brake pedal, it quiets down (barely audible). I think it's coming from the rear, but it's difficult to place exactly.

I can't really hear if it changes pitch/volume when I get on the gas because my exhaust covers it up if it does. If my exhaust noise was canceled out, I would put it at about 20dB, and 10dB with the brakes pressed.

From searching I've found it could be a fuel pump problem. Walbro was referenced in the thread, I have a stock S4 N/A one. Or it could be a differential problem, but those seem to only happen while moving (as expected).

If it was fuel pump noise, why would pressing on the brakes affect it? If it's not, wtf is it?

 

If its your whinebro, the reason pressing the brakes would change it is because you dropped your system voltage due to the load induced by the brake lights. This would be more pronounced during idle conditions.

Could be your squealers too..

 

When you press the clutch in, does it stop?
This would be the throw-out bearing. Sound would come from the front-right of the car.

Do your RPM's drop when you press the brake? If so, it could be ANYTHING. Anything that moves will slow down when your RPM's drop, so..

 

I have the stock S4 N/A pump. I just meant the thread I found was referring to a Walbro.

The clutch is in when it happens. I usually notice it when I'm at a red light in which case I usually have my clutch/brake pressed.

No. I'll double check, but don't think so.

 

Double checked and it doesn't affect my idle when I hit the brakes.

 

Is it coming from your speakers??

Could be electrical interference if the noise stops with the radio turned off (not volume off but all power).
It's a long shot but thought i'd throw it out there.

 

I don't have any. Nor is my radio even hooked up.