My 2004 Volvo S40 T5 is having a few problems. firstly, the air conditioner blows hot air when idling, but cold air while driving. Secondly, it is jerking once or a couple times when coming to a stop / braking, and while accelerating from a stop. While driving at highway speeds, it does not jerk, and runs smoothly.

With the car on, when I turn on the air conditioner, the engine revs up a little and then drops. It repeats this about ever couple seconds. It also randomly makes a whistle sound with or without the AC on, and it sounds like it's coming from my dash.

Thanks for any help!

 

Regarding the jerking, do you feel the jerk right when you apply the brake? I am thinking it may be lower control arm bushings or possibly motor mount(s). These are very common to go on these cars with age and miles.

The AC temperature seems like the system may need a recharge.

 

First as AutoNaut wrote check your AC charge. It does sound like a low charge.

Do you have a check engine light? The jerking kind of sounds like you need a tune up. Normally these cars give a warning light when it's an electrical issue. No warning light points more to a mechanical issue.