After driving for more than 3 miles, I hear a hum inside the cabin of the car. It is not audible outside the car, at all... and is most pronounced in the back seat area.. strangely enough near the back deck. It sounds like a fuel pump.. behaves kinda like one (not there with ignition off, or on (engine not running) , it stops as soon as the car is switched off...not related to road or tire speed... but the sound is only hear-able inside the cabin in the back seat. It only happens after the car has run for a while, and does not restart when the car is restarted.

I did a search and wiggled relays (DME and Fuel) and checked fuses...

I also made sure the stereo was off.. makes no difference.

The wierd thing is that there is no audible version of it outside the car.. not near the fuel pump location, or in the engine bay...

Stumped... you?
hmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm ?!?!

 

Doesn't come from your speakers? The rear glass will reflect the sound nicely... remove (not just unplug) your rear speakers and see if it goes away - a stubby screwdriver is all that's needed to undo them temporarily... noises like that drive me nuts...

 

I'm in line with garrett: I am thinking some type of vibration and/or electric noise being picked up by the rear speakers. Worth pulling them out since its easy.

 

Just for more data.. I felt the speaker cones and there was no vibration coming from them...
Could it be the power steering? It is a mid-range hum that doesn't change in frequency at all. The troubling part is that it comes on only after the car has run for a while... so hard to diagnose/repeat.

 

You gotta take out the speakers!! That's the only way to diagnose that!