When a building has both very high and low flow demands, two valves in parallel ( high/low arrangement) are used to prevent accelerated wear of the valve seat which is common when only one valve is used. Large prvs don't like repetitive low flow. So a smaller valve (designed top handle low flow) is set to a higher pressure and larger valve to a lower pressure. The small valve opens first when there is low demand (like a sink tap). As demand increases (showers say) and system pressure continues to drop, the larger valve begins to open when the system pressure reaches it's set pressure. As demand continues to increase both valves continue to open up to max flow capacity of both valves combined.
You don't have a flooding concern. But when you isolate the small valve like that and the noise stops may mean the pressure settings are off, or the small valve needs to be replaced. It would be helpful if you had pressure gauges on those valves.
(like the gym! looks like for kids?)