Side Mirror Jiggles 2002 PA

I have this odd problem on a 2002 Park Avenue. When cruising on the highway, the driver's side mirror kind of jiggles erratically. The entire mirror housing is not loose. The housing is stable, but the actual mirror jiggles. The glass does not seem to be coming unglued, yet the entire mirror jiggles. Has anyone encountered the same and, if so, how can it be fixed? I am not sure if the mirror can be removed somehow from the housing.