It is the smoke unit fan.... But if you can hear it above the typical ambient noise of a layout room it might be the bushings in the fan motor are going bad. In our club we've seen this happen a few times, and if your humming starts sounding like a 1/64 scale 737 winding up for takeoff... Call Lionel and order 1 for install and 1 to keep in reserve. Smoke fan/motor assemblies are one part they seem to always have in stock.
Don't try to order traction tires because they don't have any.
Nick C.