The only symptom is a faint hissing noise from the footwell when the engine is running which goes away with the brake pedal depressed. No, I don't keep snakes in my S4
It must've been like it since I bought the car but I only noticed it when I turned the climate control off randomly and detected a difference in the sound level in the cabin when braking. I mentioned it when dropping the car off for MOT and the technician said that whilst it wasn't necessarily dangerous I should get it replaced as soon as I could. He mentioned that a few mk2 TTs have come in with the same fault but never an A4 or S4.
The servo has been revised but isn't a cheap part (around £250), a new brake pedal is needed, and the job requires 7 hours of labour. My car is out of warranty and Audi CS weren't willing to provide any assistance with the bill, but Audi Basingstoke took kindly and helped me out with a labour contribution. Even so this wasn't a cheap job, but any issue with the braking system is a safety concern in my book so I'm glad it's sorted, and the brake pedal is noticeably more effective than before. Furthermore, any air leaks after the MAF can alter the fuel mixture and rob me of power, so on that note it was a no brainer!
So all you B8 A4/S4 owners, go out and have a listen in your footwells with the brakes on and off. Fingers crossed for all of you that there's no unwanted hissing from the servo, or from any angry snakes who may be resident
PS: The Audi tech dismantled my old servo in his own time to locate the fault. He couldn't find anything obvious but put his money on a faulty rubber seal. Here's the photos he sent through in case anyone's interested: