Vibrating And Humming Sound From Fan

hie there l drive a mercedes 92 230e w124 series, when l start the car at time there is a fan that turns right in front and causes vibrations and a humming sound. it domes and goes at intervals, it normally goes after then car engine runs for a bit, today however it was consistent, the temp did not increase however, the vibrating and humming happened even when the fan was off inside the car. what could this be , please help :banghead: