DC52 Cinetic Animal - whining noise from within the ball/motor casing.

High pitched whine emanating from the motor area (inside ball casing). Sounds similar to a blocked tube but every tube and orifice is clear. Also continues when the canister and tubes are removed. Noise stops if you block the suction (hand over intake in the top handle). Suction is however strong and the motor seems to be working well. Could it be dirty or blocked filter around the motor? Have to admit it is used in dusty conditions and is well over 5 years old. Simon (living in Italy)