Audi Repair: 200 audi a4 1.8t wired vacume noise car is off, emission control system, voltage gauge

Question
I have a 2000 audi a4 1.8t. Last week when I turned my car off I heard a vacume cleaner noise under the hood. It was not coming from the fans. Next day battery is dead. I replaced the battery and noticed at first no problems, but after a 5 mile drive the noise was going on again. When the car was turned off again you can hear a low pitched sound. Something is trying to turn over. If I do not diconnect the battery, the battery will be dead the next morning. Battery will be so wiped out that the clock or mileage gauge will not even display. This low pitched sound is also causing problems while at regular idle. When the car is idling every 20 seconds the rpms will drop maybe by 100 rpm and the voltage gauge will drop as well. then the rpm and voltage go back to normal hopefully meaning the alternator is working. Thank you so much for the help. What could be causing this loud vacume noise (when running) and the low pitched sound when the car is off? Could this be a vacume leak I am stumped.

Answer
Sounds like there is a problem with the secondary air system, which is effectively a pump that runs off the battery.  It is part of the Emission control system.  From your description the relay that operates this pump may be faulty, or perhaps the input of the realy is malfunctioning.  An import shop should be able to help you get this fixed, or if you want to check the relays, they are located under the lower dash panel.  Let me know what you find,  Jan