What is causing the hissing sound while driving but when I putnon the brakes the hissing stop?
a wheel bearing or CV joint can you describe the noise to me is it a squeal or like a leakl sound
fseal on brake pedal, brake booster. It's a common problem on Benz, see here: http://mbworld.org/forums/m-class-w163/381190-hissing-noise-under-dash-brake-pedal-area.html. AFAIK you can't buy just the seal, you'll either need to swipe one from a donor booster or buy a whole new booster. The link above shows one hack to get around replacing the seal.
I agree with KenF. Good chance a seal has failed. Suggest you try a local foreign car repair shop. They may be able to replace the seals and/or rebuild the booster at substantially less than any MB dealer.
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