Humming noise

[xCarter]

So, something interesting is going on with my 2004 A4. There is a medium-pitched hum that comes from somewhere (can't tell where) whenever the engine is on. It only goes away on warmer days (70+) and has been the same for maybe three months now. It can be heard from inside and outside the vehicle, but it's quieter outside, but I can't pinpoint where it's coming from in my casual observations. (Under the hood, walk around, etc)

The hum is not affected by:
Speed - Happens at idle all the way up.
Stopping, going - Only shutting off the engine quiets it.
Interior fans - AC, Heat, on, off, high, low, all the same.

I suspect it's something like a heat shield or other under body part thats next to something and is just touching or otherwise rubbing, but I'm not sure. Could be exhaust or the cat?

Just wondering if anyone had any suggestions.
Thanks,
Carter.

[AudiA4Turbo22]

could be a VAC leak! also check ur Fuel Filter, it could be dirty!

[sailor]

Fuel pump maybe. I have a t-bird that has a noisy pump, been noisy for the last 80k.

[Audi Skate Snow]

Maybe a fuel pump... try and pull up your rear seats and see if it gets louder.

[Carbon]

you should get the car on a lift and try to narrow it down to at least the front or rear of the car

[AudiRob]

if the sound is coming from the passenger side rear tire - then most likely its your fuel filter/pump. i replaced my fuel filter and it dissappeared.

[xCarter]

Thanks for the replies. The weather is getting warm here, so it may be awhile before I can pinpoint the issue. I'll keep and ear out!