These noises are not specific to UPP either. I've had a lot of setups on past cars, and this noise happened a lot. I always installed with the wraps or covers + some kind of anti-seizing material. I've seen that when fully unloaded, the springs would be somewhat stuck, and then pop when unseizing themselves.
I've seen this happen on Teins and Billstein coilover setups. But with the right stuff, it works just fine.