Could be a water pump seal starting to go bad. Or it could be the fan rubbing the top coolant hose. There is a bracket the top coolant hose sits in (if I remember correctly). This hose has a tendency to make contact with the fan blades over time. Look for any rubbing on the uinderside of the hose. May need to change it & re-orient the new hose so it has lots of clearance with the fan.