01 f-150 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. It is now running at the correct temp but now there is a humming or buzzing sound. When you turn off the engine the sound continues for a second and then you hear what sounds like a little metal ball dropping. I am having a a hard time determining when it is coming from but it seems to be on the passenger side of the valve cover. Any ideas?

It was just a bad PCV valve. $2 and 2 minutes later the sound is gone.