Strange noise and loss of power.

I have a 2005 cadillac cts sport/luxury. When I do a cold start the engine seems to lose power after turning over. There is a strange noise that can be heard before as the car is idling and as long as the ac is on while driving. This noise sounds like rubber rubbing something, it seems to go away after the ac is turned off. Usually when the accelerator is pressed after startup the pedal pulsates or vibrates for a few seconds then goes away after driving for a few seconds. I've also noticed that at times I can see a bit of smoke or dust coming from the hood when at a stop light but haven't been able to observe it under the hood when it happens there is also a burning smell that comes only briefly after I've driven for a couple of miles. I'm not sure what the problem is but I was told that I needed to replace the serpentine belt. What else can be possible causes of these problems? All fluids are at fill levels and the tires are good and showing now signs of serious wear.