Ticking Noise

I have a 05 gmc sierra with a 5.3. When I left for work one morning last week and my heater was not working So i watched my temp gauge as soon as it got just above 210 I pulled over and checked the coolant in it, which was full. started back up and temp was fine. When I started it to leave work the engine was making a ticking noise then went away after a few seconds. I put a new thermostat in it the next day and temp is doing fine but still getting the ticking noise from engine which is getting louder and longer everyday but still goes away after about 30 secs to 1 min. Already checked the manifold bolts. Any ideas what this may be?