When I drive with my coupe's windows down, I hear squeaks coming from the rear when the pavement is not perfectly smooth. It has increased quite a bit in the past two months.

Out of all the maintenance done to my car (CATS shocks, bushings, hoses), nothing has been done to the rear suspension and all visible bushings look ok. I was planning to replace all the bushings and likely go with a larger rear anti-roll bar anyway so.... ideas?