Why do I hear fan running after I turn engine off? It stops after 20-30 seconds

What is your question?

It’s like a humming noise or a fan running type noise, stops after 20 +/- seconds. Is it something to do with the DEF?

What makes this problem better or worse?

I don’t see anything that make it worse

How long has your 2011 Ford F-250 Super Duty had this problem?

Just bought it 2 weeks ago and I’ve been hearing it since, someone mentioned its the DEF flowing back in