I have an 04 S4 and have experienced a low rumble/large vibration after accelerating hard. It seems definitely related to the clutch and if I ease off driving hard for a while, it goes away. It happens in all gears. It might not happen if I have been driving the car and using the clutch in traffic? not sure. It never does it in normal driving. The car has 90K and has been dealer maintained all it's life. As I read posts I hear wheel bearing, throw out bearing, engine mount, a suspension issue, engine mount, Glazing on the FW, CV joint, or clutch. Any help?

I would say take a look at the tranny mount for any cracking.

After you accelerate hard are you engine braking or is the clutch depressed?

i have only noticed when accelerating hard (an maybe when the clutch is not up to operating temp). when i push the clutch in though (disengaging it) it stops. Can you help me locate the tranny mount? i am new to audis. thanks.