My wife's 1998 A4 Quattro w/ V6 is having an issue with some component not shutting off. It's comming from the front right (pasenger) side of the engine. The sound can be heard here: https://www.youtube.com/watch?v=-l6N...ature=youtu.be Whenever we take it to the mechanic it will not do this. This thing is draining the battery in a few hours.

 

That almost sounds like the starter just spinning at full speed (but not engaging the fly wheel). Only thing I can think of that would be that loud in the engine bay.

 

What would cause the starter to spin by itself with the key off?

 

Okay, not sure about on an Audi, but I know starters have a solinoid that tells the starter when to engage. If that has gone bad it could cause the starter to just free spin. There could also be a short in the wiring from the battery to the starter. Does the car start okay? If it does start do you still hear the noise?

Another item it could be (just normally wouldn't be that loud) is if your cooling fans are not shutting off. They are suppose to cut off after the temp gets to a certain level. Mine runs for just a few seconds after shutting the car off. This is easy to check. Start the car with the hood open and see if the fan is spinning and making the noise. Then turn the car off and see if the fan(s) stay on (or starts up) for more than a few seconds to a minute. If they do you might have an issue there. There is a sensor that tells the fans when to turn on and off. Not sure of the cost to replace.