Audi S4's mystery hiss points to vacuum leak fix needed

Had similar symptoms on my 2015 Audi S5 last winter. The increased fuel economy and hissing noise pointed to an air intake leak, which was causing the engine to run rich to compensate for unmetered air entering the system. Initial diagnosis revealed a torn intake hose between the air filter housing and throttle body. The engine misfire became more noticeable under load, particularly during acceleration. The issue became serious because unfiltered air was entering the engine, potentially causing long-term damage. The repair involved: Complete inspection of all vacuum lines, Replacement of cracked intake hose, Cleaning the throttle body while it was accessible, Checking intake manifold seals, Resetting engine management system Important to address this quickly as it affects both performance and engine health. The torn hose was allowing debris into the system, which could have led to more extensive engine damage if left unchecked. Would recommend checking all rubber hoses in the intake system for cracks or splits before visiting a shop. Also inspect around the intake manifold gaskets for any obvious air leaks. These checks can help pinpoint the exact issue.

Thanks for the detailed response on the intake system issue. My problem turned out very similar, vacuum leak from a cracked hose causing the turbocharger to make that distinct hissing sound. The check engine light was definitely a useful warning sign. The leak detection test confirmed it was a failing rubber hose near the air filter housing. Wanted to ask, how has your repair held up since then? Any other issues pop up related to the intake system? Also curious about the final cost of your fix, as my shop quoted quite high and wondering if it was reasonable.

Since getting the air intake system leak fixed at the shop, everything has been running perfectly. The repair has held up great with no additional vacuum hose issues or check engine warnings. The turbocharger is working smoothly now without that concerning hissing sound. The total cost came to 100 Euro for replacing the leaking intake hose and performing the full leak detection test. This seems reasonable compared to what others have paid for similar repairs. The price included checking all connecting hoses and ensuring proper seal around the air filter housing. One tip, I now regularly inspect the rubber components of the air intake system during basic maintenance. This preventive check helps catch any developing issues before they trigger engine performance problems or warning lights. The improved fuel economy and restored engine performance definitely justified the repair cost. No related problems have emerged in the months since the fix.

Thanks everyone for the helpful input. I took my S4 to a different shop this time and had a much better experience. The mechanic found a cracked vacuum hose that was causing the turbocharger issues, just as suggested here. The repair was straightforward, they replaced the damaged hose, performed a full check of the air intake system, and cleaned the surrounding components. Total cost came to 180 Euro which seemed fair given the diagnostic work and parts. The hissing noise is completely gone now and fuel economy is back to normal. No more check engine light either. They also showed me how to spot early signs of wear on the intake hoses for future reference. Really glad I asked here first, saved me from getting overcharged again and helped identify the actual problem. The car is running great now, feels like it has its full power back during acceleration.