An improved centrifuge. The centrifuge may be a conventional centrifuge having a rotor and a plurality of tube receiving adaptors inserted into the rotor in a circular configuration. The improvement comprises a flexible flap having a fixed end mounted on the rotor at an adaptor and having an opposing free end. The free end extends radially inward toward the center of the rotor when the rotor is not rotating and is folded by centrifugal force back to cover the opening in the adaptor when the rotor is rotating. By covering the opening in the adaptor, the flexible flap eliminates the high pitch whistle otherwise generated by the adaptor when rotating without a tube inserted.