01 ford f-150 5.4L Humming noise after changing thermostat
 

My rig was running cold and the check engine light came on. I got the code and it said it was the thermostat. I bought and changed the thermostat. Now there is a humming or buzzing sound coming from the engine. When you turn off the engine the sound continues for a second and then you hear what sounds like a little metal ball dropping. Any ideas?

Well, always start with what you worked on! Check that the new t-stat is opening, that the housing isn't warped, and general cooling system performance. The noise is most likely a water hammer type concern from a disturbance in the coolant flow within the system.
Does the upper hose feel warm and the lower on much cooler after runing the truck? Have you bled the system to make sure the air is out?
Let us know...

01 ford f-150 5.4L Humming noise after changing thermostat
 

I was wondering about bleeding the air out of the system. How do I do that? I appreciate the input.

In this application, leave the filler cap loose on the tank, run until thouroughly warm (when the t-stat opens), add fluid until full, run a little longer, check the water level. Install cap firmly. Shut it off, and watch the coolant level; let the engine cooldown (overnight would be best), and add more fluid if necessary. Go and drive it, and tell us what breaks!

I tried the bleed with no change in the noise. Thanks for the help. I decided to keep moving my hand around in the engine to see if I could find the noise by feel since by ear it wasn't happening. It was a bad PCV valve. I do love a $2 fix. Sounds great now.