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.

3 answers
COULD BE IDLER BEARING GOING BAD AND ONLY MAKES NOSE WHEN UNDER A/C LOAD
Also have the ac compressor belt checked could be loose or defective. Has it's own belt.
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/