2007 Chevy Monte Carlo noise coming from rear end

Sometimes not every time when I accelerate from a stop I hear what sounds something like a nocking sound from the rear of the car. It sounds like something is tightening up while it happens. It is never more that three nocks each time. It seems to hapen more when it is cold and wet outside. I am thinking it may be either the tracktion control system or the antilock brack system. I have not taken it to the dealer yet because it does not happen every time. I was hopeing to get some kind of idea of what it might be before taking it to them.
Thanks for the help.