'Fraid you've got all the symptoms of a broken stud .I had this last year on mine started with a light click which like yours disapeared once norm running temp. was reached. After a while it got so that it was audible continually irrespective of temp. I was told by local GM dealer that it could be run like this with no damage but I couldn't stand that bloody ticking.......
I investigated it and found that the first two studs top and bottom were sheared .I considered doing it myself but did not have a drill small enough to get between the head and the inner wing ( the garage use a 90 degree drive drill ). I also found out that one of the studs is only a couple of mm from the water jacket so the decision was made to have it done by the GM garage. They burst through ,they replace .
When they did it the exhaust manifold was cracked. They e mailed me a photo to ask permission to repair
.This was done by a local agricultural engineering company as welding castings is a specialist process. They also had to take the studs out to 10mm from the puny 8mm standard item. They also fitted the later improved gasket as recommended by GM, flushed the cooling system through and topped it up with fresh antifreeze to the correct mix.
It cost me about 400 euro ( approx £300 ) which I didn't mind as it was done right with no further problems and believe me when I tell you they made me a lovely cup of tea when I was paying the bill!. I can't understand why this is such a common problem on all the ecotec engines ,it seems to always be the two studs I mention on in line and transverse engines, the garage suggested that it was a poor design of gasket originally but I wonder if there is some kind of localised high frequency vibration from the cam bearings at that point. Who knows...
Anyway , I hope I'm wrong by a mile and you find a penny rattling somewhere in the engine bay....
Cheers
Don