The BL-91 is a vintage TT. Begin by checking all mounts. It's servo section could be off - electrolytic capacitors have a finite lifetime - and cause noise. I would begin by checking all the components in the servo board, then try to find DC in your mains supply (the BL-91 uses a gapless U-core xformer which goes south with only 500 mV DC). Then the motor should be checked, but it's DC, not AC, and shouldn't be audibly noisy in most circumstaces. My bet would be defective 'lytics in the servo section and/or mains pollution.
