I have an 04 basic. When I turn on the ac a humming noise comes from the rear of the car. Its really loud in the trunk and when I touch the floor of the trunk, you can feel the vibration. Its there sometimes when the car runs without the ac, but much more predominant and noticeable when the ac is crnked up. Also, there is a spitting type noise or hissing from the rear outside. It sounds like it might be coming from the exhaust somewhere. When I reverse, there is a clunk noise that comes from the rear too when I hit the brakes to shift into forward motion. This car is becoming such a headache for only owning it for 8 months. The cv axles need to be replaced, the control arms are starting to give out and it pulls to the right real bad, two engine mounts are bad, it leaks coolant from somewhere and I just had the whole system replaced in december. And to kick it all up a notch, I still owe 14k on it.

 

dang...


I think the hissing from the rear is the fuel filter...needs to be changed

 

The joys of owning a German car, you need to keep up with the maintenance.

Your hissing noise is your fuel filter/pump (replace the filter first)
Your Axles need to be changed ASAP, as well as your control arms, that's the reason your car pulls
Your spitting sound, mine does the same but I have an anfter market exhaust and a K&N filter in the intake.
You're looking @ around 2 to 2 1/2 K USD for just parts.

Don't fall behind on maintenance or you're going to keep finding nasty surprises like these.

Your coolant leak is more than likely your Coolant crossover pipe.