Car tire making noise?? serious answers only please!!!?

About 3 weeks ago i started getting an odd screeching noise from my front left tire when i reverse and now i notice it a little bit when i make sharp left turns also. It never happens while im just driving straight or making little turns. I have an AWD SUV if that matters at all. It almost sounds like metal is rubbing in there. I know i am due to get my brake pads replaced and rotors resurfaced. Could rust be the cause of this noise? As i mentioned, its only in the front left tire. I have heard it could be low tire pressure, but it doesn't look any lower than the other tires. i cant really even figure out how to describe the noise. almost like loud nails on a chalkboard or like a metallic screeching that needs to be lubricated. Any serious answers would be appreciated. If its something where repair is costly, can you also tell me how much so i know what to expect if i have to take it to a mechanic to be fixed.