High pitched hum

[TeamRinaldi]

Ok, so I've got a new sound. You all know what I mean, you know every sound and squeak your truck makes. Then one day, there's a new one. Hopefully one of you guys can point me in the right direction. It's coming from somewhere on the front left side. I can hear it sitting in the cab and when I crouch down beside the tire, but when I pop the hood I can't hear it. At first I thought power steering fluid, but that wasn't it. It's a real high squeal, like a mosquito in your ear. Any takers?