Whistle boards are typically placed ahead of crossing points with no barriers. In some cases that might be where a very rarely used public footpath crosses a rail line. Even though only a few people might use the crossing over a period of many months, drivers are still obliged to sound the horn on every approach. (The only exception would be in areas where Network Rail has authorised trains to pass that point,without using their horns, during certain night-time hours).
Thanks all, they must be using their horns to warn workers near the bridges. That or there is a W sign near my home. I'm not complaining i like the noises