When i turn on the cold tap on the bathroom sink, and as soon as the flow rate i want is reached (half a second), the tap makes one loud bang under the floorboards (like the bangs when the water's just been turned on and theres air in the pipes still being forced out of the tap), and the water flow rate drops to a fine trickle, i have to turn the tap another whole turn to get the flow i want. Whats going on????