I've had this issue since the day I got my GT2, completely intermittent and incredibly annoying.
At first I thought it was related to getting a tint on my sunroof. But I took the tint off and proved that wasn't it . Some days it isn't there and I forget all about it, other days it sounds like the sunroof is coming apart.
I feel like it is more related to direct sunlight as opposed to temperature, but I don't know. Hard to troubleshoot something that is very random.
Side note, also get a single loud creak from the rear hatch glass if I go over a speed bump at an angle, this happens at <5mph basically every time