2002 - 4Runner - 150,000 miles
I drove about 350 miles today, no issues. We parked and about an hour later hopped back in. When I started the truck it sounded like an airplane taking off under my hood. The heat and a/c were off. When I started to drive it subsided and finally died out but came back the next time I started it up. I am on the road so a bit worried about what it could be. Any ideas?

 

Google "fan clutch" and you will have your answer.

Its 100% normal.