What is the high-pitched whistling noise when my a/c is turned on? on 2007 Hyundai Elantra

The a/c works fine but when it becomes hot outside, the noise gets louder. The max a/c setting on the knob does not work at all either. As you accelerate, the noise gets louder as well.

Asked by for the 2007 Hyundai Elantra
Also have the ac compressor belt checked could be loose or defective. Has it's own belt.
2 more answers
COULD BE IDLER BEARING GOING BAD AND ONLY MAKES NOSE WHEN UNDER A/C LOAD
It could be your sway bar. If it is snapped it will make a thumping noise when you steer and will make no noise when going straight. It needs to be fixed immediately.http://www.lightdeal.co.uk/