CPU fan smell like burn?

There is no way that your computer should be emitting any smells under normal cirstances. And a crackling sound is also a clear indicator something is wrong. I would check the internal temperature while it is under load. Also, do something with it to put the system under stress, like playing a game or watching a movie, but watch the inside of the pc for any irregularities. Having the side already open makes this easy to do. If/when you hear the noise, check each part to see where it is coming from. If it doesn't happen again, though, then it may have been an isolated incident.