Is there aluminum wiring in my vent fan?
Sometimes with stranded wire, they will tin the ends with solder.  I would guess this is the case here since you're right that aluminum is not usually used. I think the only reason they do this is to keep the ends from fraying and keep it neat. 

You can check for sure by stripping new ends. 

I have never seen stranded aluminum, or anything less than 12awg used (most house wire is 14awg copper, which is a bit smaller). You can see in this picture the aluminum is slightly bigger:



I would also be very surprised to see aluminum wire since it requires special wire nuts (co/alr rated) and an anti-oxidant paste (eg, Noalox) to connect.

If it's just tinned copper, you can twist and wire nut to a regular standed wire, just treat it like a normal copper wire.