A/c compressor dodge ram 1500 help?

I have a 2004 dodge ram and when I turn on the ac there is a knocking sound. I took it to a mechanic and also had the dealership in which I bought the truck off of check it out. They both told me the clutch was engaging it and its supposed to do that. A friend of mine tells me the serpentine belt can come loose and leave me stranded. Who's right? I also had a 3 rd person tell me it's good that it's making that noise. I still not so convinced.