[W126 Coupe] where is that smell coming from?

[Rico2005]

(Please cc me on replies as I am not receiving list updates)

Hi, I've been driving a bit on my Euro 560SEC (it has been raining here
lately, and I don't usually drive it in rain), and have noticed an odd
smell that occasionally permeates through the passenger cabin. I say "odd"
since I can't quite pinpoint the smell. I have replaced my EHA valve and
seals recently, as well as my gas cap. But honestly, it isn't a "pure gas"
smell. Strangely enough, it smells more like propane than anything else.
And no, it wasn't when I was following a propane-powered vehicle.

I asked my mechanic about it, and he wasn't sure either. He suggested
changing my fuel to see if that had any effect (I use Chevron 92 octane).
But my windows and such seal pretty well, so I can't imagine it being the 
exhaust coming back in. So I'm a little perplexed. Appreciate any thoughts.

Rich
'87 Euro 560SEC
http://qube.myvnc.com/mercedes/560SEC/page4.html