I have the same problem and I have yet to diagnose and fix it. My park plugs on Cylinders 4,6 and 8 are fouling. I was told by the dealer I need a valve job as oil is getting into the combustion chamber. I have heard that it could also be a fuel distribution problem, or another oil delivery problem. I am going to get it checked out and hopefully fixed in the next month. I will let you know what turns up.
PS I have the same hissing sound from the rear passenger side wheel well. I posted on it a few months back and heard from several people that said they had it on their w126s too, but had no other running problems they could associate with it. Let us know what you find out about it.
1995 E420 - retired
1986 420SEL - retired