Hot water flowing into roof tank

Hello, Apologies if this doesn't contain all necessary info. Recently moved into a new house (not a new build, just new to us!) and had hot water cylinder installed. Shortly after, we have noticed that when the hot water advance is activated, there is a sound coming from the roof I hadn't noticed before. Looking, it looks like there is a cold water tank and this other tank that is being filled up with super hot water only when the hot water advance is on. I have a video but don't think I can attach here. Any advice if I need to go back to plumber or if this is normal would be gratefully appreciated!