On the Quattro version of the A4, the gas tank is not flat. It is shaped and mounted in the A4 like an upside down U. This allows room for the drive shaft to meet the rear differential. The fuel pump is only on one side of the tank. So, how does the fuel get from one side of the tank to the side that has the fuel pump?

According to Shumburg Audi, there is an injector that picks up the fuel and sends it over to the other side of the tank (where the fuel pump is).

Shumburg stated that the injector (not to be confused with a pump according to Shumburg) is probable making this sound and if the car starts and runs fine, do not worry about it.

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