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Knocking sound from rear, no engine start required

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I hear a knocking sound for a few seconds from the rear of my 2004 Audi S4 when I turn the key to the on position. It is independent of the engine, this knocking occurs with or without starting the engine. Here is a recording of the sound: youtu.be/dxujW1LQztc

Any ideas as to what could be making that noise?

My gues is that it is the fuel pump. The fuel pump is in the gasoline tank which is at the rear end of the car. It may be on its way out.

My thoughts are the same as Triedaq’s.
Rather than winding up stranded in an inconvenient place, at an inconvenient time, with an inoperative fuel pump, I would suggest that you have your mechanic check this possibility a.s.a.p.

Triedaq, I tip my hat to you. The fuel pump never even occurred to me.

Having the sound be there on the key turn makes me think fuel pump. There’s usually nothing else mechanical that’s powered at the rear. This car doesn’t have air springs does it?

The car does not have air springs. Thank you to those who mentioned the fuel pump, I also had not thought of that.

Update: I had my regular mechanic listen to the recording and he thought right away that it was the fuel filter. He suspects the sound is from the pressure regulator (or maybe it was just a check valve) inside the filter. I’ll follow up with the results of replacing this filter.