smells like burning plastic when I turned on my heat?

[fatpwrtouge]

Hey everyone. Been here for a while just searching around and reading threads etc.. Anyway, this is my first thread here, and I've done some searching but can't pin point my problems from other HVAC threads. I've also checked a few other car forums for similar problems but they ended up with no help.

Car Info:
1997 318ti
137k miles
Automatic

Heres my problem.
Just starting tonight, after I let my car warmed up and turned the heat on. There was a strong burning plastic smell coming from my vents. Once I got up to 35-45mph the smell was less noticeable, almost as if it wasn't there.

But when I was going roughly 30mph and slower, the burning plastic smell was more noticeable? I've check under my car to see if there was anything obvious as to plastic bags etc... and found nothing. I've popped my hood and checked around the engine bay and found nothing noticeable either. Nothing looked melted or burnt from a visual inspection. I'm a bit loss. I wasn't able to look clearly behind the engine/bulk head, but nothing was smoking out of the engine bay. And I check everything with the car idling.

This just happened after a cold start also, wasn't there before I parked the car for roughly 7hrs.

I'll be looking around the forum still and try googling if I can get any answers. Until then, hopefully someone can help me with the problem if they've experience the same thing. Thanks.