A motorist struck a bad chord with police when he was caught playing the trumpet while driving over Melbourne's West Gate Bridge.

The 27-year-old man from Portarlington was intercepted by Highway Patrol after it received a number of calls about a black sedan driving erratically towards the city last Friday afternoon.

Police managed to catch up with the car and pulled alongside it to check on the driver.

To their shock, they saw the man was playing the brass instrument as he drove.

The driver was handed a $354 fine for careless driving.