Vibrating in front passenger door

have this vibrating comming from my front letf speaker area, had Vauxhall look at it 3 times now and had the speaker replaced, but still there and they say there is nothing they can do. as Im not a car guru i dont want to start taking the door off. Any ideas what it could be,
when i put the radio volume up it gets worse and the sound is still clear just someting vibrating.(Have bose suround sound in the car)