Some fans rattle or groan on low speed. This is caused by moisture and lack of bearing lube. A product from WURTH called HHS2000 sprayed into the bearings can alleviate this problem for a long time...and it's much cheaper than a new fan that you don't really need yet.
Take out the glovebox -- look for the screws around the perimeter behind the glovebox door. You'll then see the fan mounting on the air plenum. Unscrew these screws and disconnect the power leads. Now, with a lot of wiggling, prying, and forcing, you should get the old fan out (there is just a little lack of room between the fan mounting and the local brickabrack). For a little clearance, remove the dash screws on the passenger side next to the A-pillar. These are under the little round caps. This will allow you to move the dash out slightly, making it much easier to pull the fan out.
Take a page from Audi Dudi's book "if it won't come out, remove something else". The trick is to remove the fan from the housing which holds it. Two screws hold the fan to a plastic housing. Remove these, pull the plastic housing off, leaving the fan inside the plenum. With a minor amount of wiggling, the housing will now come out. Then you are left with the motor and blower. They will also come out. This housing normally comes off with the fan, and taking it off first gives you just enough clearance. Reverse the procedure to install your new fan.
