Furnace cold air return serving up crawlspace air
Fact: ducts leak

Just about all ductwork leaks to some extent; in fact, without specialized installation techniques, leakages of around 20% can be expected.  Combine that with the fact that the furnace is sucking on the return ducts, and you get a recipe for air infiltration from the crawlspace.

Venting the crawlspace might help, but sealing the ducts is better all-around, and rerouting them is even better yet

As far as the smell goes, venting the crawlspace might help, but a better bet is to have the ducts in the crawlspace redone in a sealed fashion -- it's possible to get the leakage down to a few % this way.  Of course, it's even better to reroute the ductwork so it's in the conditioned space instead, but not all houses lend themselves to that approach.