my truck has been doing this wierd humming noise from the engine bay. My Toyota tech says it is coming from the alternator. Has anyone experienced this before and does it mean the alt is about to go?

 

Do you hear the humming on the inside of the truck or only when you are listening under the hood? If you are only under the hood and hear it, could the pulley on the alternator have a bad bearing? Just a guess. If you hear it on the inside of your rig, could you maybe have a ground loop ( I think that is what it's called) caused by a larger stereo install? Just a couple guess to try out.

 

In my experience a hum or groan from under the hood is either low fluid in the power steering or a bad diode in the alternator. The sound is similar. Check the fluid in the PS, if OK, pull alternator and take to a rebuild shop and ask them to check it out. I suppose it is possible it is an alternator bearing.

Let us know how this plays out.