peter wrote:
If I close my bathroom door and then turn on the ventilation fan, the air
being sucked out of the heating vent has a crawlspace odor (I've been down
there many times, so I recognize the smell).

I suspect there is a leak in the heating ducts in the crawlspace. However,
when I went down there to take a quick look around, I didn't see anything
obvious.

That crawlspace smell might also be mold growing in your ducts. Have
you had them cleaned?

R