This happens on my 87 300E from time to time. In my car, it also gets louder when you step on the gas, and then seems to quiet down when you release it, although sometimes it can get very loud without any pedal movement, and I've had it show up when the engine is off (!) too.

I discovered a little while ago that if I stop the car, walk around to the rear, and crack open the gas tank cap for a second, it seems to depressurize the fuel tank and the whining stops. I don't know what this means, or why it does that, but thought that it might be helpful.