The Sigma Centrifuge makes a loud noise at low speeds which disappears at higher speeds, when using the 2x96-well plate rotor. What is the problem?
FAQ ID -776

The pivot points between the swing-out buckets and the rotor need to be lubricated to ensure that the buckets swing freely as the centrifuge changes speed. Spread the grease lightly on the rotor where it contacts the bucket, as frequently as needed. The lubricating grease can be ordered from QIAGEN using part number 9238680.