Strange Noise Under Truck Hours After Driven

I had driven my truck several hours ago and decided to grease the U-joints. I got the first one done and was going to do the second, when I realized I was out of grease in my gun. I hopped in my wife's car and ran to Autozone to pick up another cartridge and run another errand and when I pulled into the garage I heard a strange whirring sound. I realized it was coming from my truck and after some searching, it was coming from the part in the attached photo. I had never heard it before, the keys weren't in the truck and it had been several hours since it was last driven. I was gone about 30min, so I have no idea how long it had been doing that, but it stopped about a minute after I got home. Any ideas as to what it is?

 

That's normal, it's your emissions system performing a self check. Mine does it too every now and again. It's not every shutoff, the truck just decides to do it sometimes.

This is a good read for you: https://www.tacomaworld.com/threads/wtf-is-this-sound-from-rear-end-truck-is-off.308857/

 

Thanks, guys!