Lately, there has been a really loud humming noise coming from the AC unit of my RX 300. It goes away after a few minutes or if l turn on the defrost. Also if the AC is on when l start the car there is a squicking belt sound that goes away as soon as l turn off the AC.Does anyone have an idea what this might be from?

this could be that you car needs a new A/C compressor when you start your car up your compressor does not want to turn which is forcing the belt to slip around the the A/C compressors pulley wheel. If it is your compressor than your looking at $600 or more to have it replaced or if you could just have it bypassed for now but than you would not have A/C or defroster.

it may be your ac pullys the bearings somtimes squeak

Actually sounds like you have two issues.

1st - Humming sound could be from the blower motor. Remove and check for dirt and leaves in the housing or fan blades.

2nd - Inspect your fan belt, it could be glazed and need replacing. Does the system blow cold? If not the worst case scenario is the A/C compressor is locked frozen or the clutch is worn out.

Take your overpriced Rav4 to the dealer, you can afford it since you pay way too much for that thing.