hissing sound coming from rear under carriage

I noticed this yesterday on my 1990 190E. While my engine was running, I heard this hissing sound coming from the rear. At first I thought it was a tire leaking air. It wasn't. I went up to the front to see if it was coming from the engine bay. Not there either. When I stuck my head near the driver-side rear under carriage (near the rear tire), I heard the noise coming somewhere from there. I have no clue what it is or where exactly it is coming from. BTW, the hissing sound was noticed when the car was warming up from sitting in the garage for a few weeks. The hissing is only heard if I'm outside near the rear of the car. And very noticeable if I put my head down by the under carriage.

After driving my car and fully warmed, the noise was either gone or perhaps still there but not detectable. Anyone know if this is normal for a 190E or any Mercedes? I'm not too worried because I think the hissing has always been there, but I just recently took great notice of it.