I have a DC39 which has recently suffered a loss of suction after working OK for some years. I have to date replaced the motor pre-filter and the hepa filter and cleaned the cyclone unit and hoses and turbo head which are all free from obstruction. There seems to be reasonable suction from the head part where it attaches to the pre-filter but little at the end of the hose when the rest is assembled. I have noticed that the is a noise from the motor when the suction is obstructed by a hand which sounds like a flap engaging or moving. This sound is repeated when the motor is turned off as if the 'flap' is returning to its original position. Is there a bypass valve in the motor which is being activated? I am at a loss as to the cause of the problem so any help or advice would be appreciated.