Audi Repair Questions? Ask a Mechanic for Answers ASAP
Hello, my name is John.
The musty smell can come from inside the air ducts. There is a product I use in my shop made by BG called Frigi-Fresh. There is probably a similar product available if your local auto parts store does not carry BG products. The Frigi-Fresh is sprayed into the air intake duct (at the base of the windshield, outside) and circulates through the air duct system. Let it soak in a while then run the fan to clear it out. There will be instructions on the can.
The A/C drain is not hard to clean and can be done with a length of wire. The trick part is locating the drain underneath the car. This is much easier with the car on a lift. The drain outlet is typically found under the car, near the firewall and slightly to the passenger side from center. There are other reasons for water entering the car in heavy rains such as clogged drain pipes for the sunroof if you have a sunroof. Or possibly if the rain was heavy enough it may have exceeded the drain tube capacity to drain and overflowed in the car. If you run the A/C on a dry day and you hear sloshing behind the dash or have leaking into the car then that tells you the drain is plugged. If that does not happen then the drain is clear.
Good luck, let me know how it turns out.
The freash air intake is at the base of the windshield, from the center to the passenger side. With the system running (fan on high) just spray along the base of the windshield (easier with the hood open) and the product will be drawn into the system.
There is no drain plug for the evaporator drain, it is kind of a one way valve further up in the system. If you can find it just use a lenght of wire to push up in there and clear it out.
When looking for a diagram I found this service bulletin for your vehicle. Let me know if the link does not work for you.