Mercedes_300

Here is my situation. Just bought a 2015 C300 with 40K miles with the "Luxury Package. Just had the car in for work for oil leaking from the crankshaft. To learn that the car I bought a week ago "in-service" date is now expired as my 4 years is up before 50K. So I had a nice bill of $650. Besides the fact a few days after I bought the car I noticed a brisk air sound coming from the break pedal are when I lift up off it. First few days I thought it was my shoe brushing the break pedal as it was so quiet. Then I noticed it more so in stop and go traffic. The "hissing" sound would last 1-3 seconds from depressing the break quickly. When I would slowly press down and let up it would not make the sound. So fast forward to when I took it in for my leaking seal I told them I think I might have a bad break booster. The mechanic took my car and could not "replicate" the sound and further inspected the breaks and mentioned the rotors and pads are brand new! Ok.. great, that does not explain my hissing noise. Is this normal? They just shook it off like it was something they hear all the time. When I got home I proceeded to watch a youtube video to check if my booster is bad. They said to turn off the car in park, pump the breaks until it gets hard, the keep foot on break and start the car.. Well my foot depressed when I turned the car on as it should like normal... I'm stumped

Is this sound normal? I might add I am in WI and it was been around 20 degrees ever since I've owned the car. The breaks seem to work fine, no lights are on, service and inspection was just completed.

Mercedes_300

I will add that this hissing sound does not happen when the car is in park, seems to only "hiss" when the vehicle is in motion.