I have a 2007 Scion tC. It just started making a weird deep whistling/humming noise and It started getting really hot under the hood but the temperature gauge isn't moving up it is in the middle. Any ideas on what it could be?

 

does it vary with engine speed? if so, first thing I would check is the alternator. check voltage at battery when idling. should be around 13-14 volts. when the diode pack goes it makes a very distinct whistling/squealing sound. may or may not be that, but it is pretty easy to check and eliminate from the list.