Whistling tow package mirrors

Anyone else have a rediculous mirror whistle? I have an '08 LE 4x4 with the extendable towing mirrors and they sing at 40 MPH or more. The only thing that quiets them is electrical tape over the slots that enable the mirror arms to pivot. The dealership says that the noise is consistent with others on their lot and therefore, is just the way it is. Not a major problem as the tape lasts for a couple of months at a time and is inconspicuous due to the black mirror base. Just wondering.