The other day I was driving home and I kept getting whiffs of something burning. I thought, this is great, now I am having the mystery smells coming from my car. I also kindof blew it off as being another car, or someone burning garbage or something. I pulled in the driveway and smelled it stronger and I knew it was my car then. I got out and raised the hood and really smelled it good. It smelled like plastics. I looked and looked and smelled around, thinking that I would probably never find it, unless it started blazing. Anyway, I followed my nose and tracked it down to the rear part of the exhaust manifold on the driverside. I discovered that some of the underhood insulation (which seems to consist of foam and a plastic coating - at least on the 80s S-Class) had falled down on the manifold and had melted to the manifold. I thought this is great. I removed as much from there as I could. Hopefully that smell does not linger about. I pulled off the other loose insulation under the hood.

So, if you smell an unusual smell under your hood- also check for fallen underhood insulation on your exhaust manifolds.