We get the same racket from the right side (viewed from the cab, LHD) like hippo, what with that and all the other rattles and bangs, listening to the radio etc is a waste of time. When the satnav is on it is a constant "what did she say ?" I have improved this a little by screwing down a square of perspex halfway back on the carpet dashboard where the gps suction cup sticks ok, and she doesn't have to shout so loud !
I also have tried just about everything without success, it's the same place - somewhere a few inches down from the top front of the side window. It isn't near the big mirror and anyway it would whistle on both sides if it were the mirrors and it doesn't
. Hope you (we) find the solution