Related to the original topic and also equally important for cabin air quality, check the bulk head seal for gaps or lack of composition.
i have seen a few engine bays with no bonnet liner, this too is also important as it forms the seal with bulk head - if either of these two are in need of repair engine fumes can enter directly through blower intake which sits just behind, or at ;east on the MB's I have worked on (possible W116's as above are different)
Bulk head rubber to the rear of engine compartment.
