I recently replaced an old Opella Torbeck side entry WC cistern filling valve with a new identical one. Flushing and refilling proceed normally but at the instant the valve cuts off after filling it emits an ear-splitting sound exactly like a high-pitched whoopee cushion (funny at first but the novelty has now worn off). This lasts for 2 to 3 seconds and can be heard two floors down! Thinking it was a faulty valve I replaced it with another identical and new one, only to get the same problem. I emphasize the sound appears to be coming from the filling valve itself (not the pipework) and only happens at the instant when the valve shuts off after filling. Any suggestions would be welcome.