(06 Apr 2016, 4:29 am)Dan wrote That's the low bridge warning sound. If a bus is within a certain radius of a low bridge, there will be a warning sound of some sort (there's various types depending on the bus - some beep, some shout 'WARNING', etc).

This serves as an alert to the driver, and makes them aware of the potential hazard.

Oh, I've seen them on other services GNE services before. They are made by Hanover if I'm not mistaken.