Mercedes M Klasse Hybrid Hissing Sound and Fuel Economy Problem

That sounds familiar! I had almost the same thing happen with my M Klasse a while back. That hissing sound is a dead giveaway. If your check engine light is on, definitely get the codes read. It probably is a vacuum hose. Those things get brittle and crack over time. Check especially around where the intake hose connects to the engine. How's the engine running otherwise?

When you had the issue, did the hissing sound only occur during acceleration, or was it present at idle as well? Did you perform the repair yourself, or did you take it to a professional? If you did it yourself, any tips on locating the leak? I was planning on visually inspecting all the hoses, but any specific areas to focus on would be greatly appreciated.

Mine was mostly noticeable during acceleration, but I could hear a faint hiss at idle if I really listened for it. Honestly, I tried to fix it myself for a bit, but I ended up just taking it to my mechanic. Sometimes it's worth paying someone who knows what they are doing. I'd say if you can't spot it easily, just bring it in. They have tools for leak detection that make it a breeze. Don't want to mess anything up further. Could end up costing you more!

I took your suggestion and brought it to a trusted mechanic. They confirmed it was indeed a leaking intake hose. They replaced it, cleared the check engine light, and now the hissing sound is gone. Fuel economy is back to normal. The total repair cost was around 100€, which was reasonable. I really appreciate your help in pointing me in the right direction!