I have had the same problem in the past, kinda smells like burning plastic/chemicals. Happens on my cooker when I run hot. I have cleaned, inspected welds, tried different wood etc... still no solution. I now just leave my firebox door open when cooking hot and that seems to put a band-aid on it. However i would like a real solution as well.
Meadow Creek TS250, Horizon 20" Classic Backyard Smoker, old style weber performer