While looking at purchasing a used 1998 C280 with 73K miles I noticed that the heater fan would emit a vibration noise from position 3 (on the digi display) and get worse as the fan speed was increased. It was almost like a thumping, or shuttering vibration noise. Any idea what this could be except for 'expensive' (remove dash and replace fan etc.)? Should I avoid this car and look elsewhere?

Thank, MarkC