You should start by checking your coolant level, it could be low allowing air to enter the system causing the noises you describe. If it's low, fill with antifreweze and water mixed 50/50. Check on a daily basis to see if coolant level drops, some drop may be normal if it was low, but after a few days, you should start looking for a possible leak.
Let me know if you have more problems with this.
Alright, at least that is ruled out. Have you noticed any driveability problems, such as hesitation or lack of power? Any excessive fuel consumption that you've noticed? Will the sound disappear if you hit the gas pedal harder? And, are you up-to-date on the routine maintenence?
And this is the harder part, and I realize how difficult it will be, but could you describe the sounds a little more? Tough to do in a text enviroment, I apologize, but it will really help in my finding the potential problem(s).