We have a Durgo valve fitted to the en-suite loo in our loft. It's boxed in but doesn't have the "air holes in the boxing" which the installation instructions recommend. Could this be causing the rumbling noise, very similar to knocking pipes, that we experience every time the loo is flushed and would creating the "air hole" cure this? Putting air holes in the boxing shouldn't let smells into the room should it - the Air Admittance Valve is there to stop this isn't it?
The durgo valve is designed to allow air into the pipework, not to allow foul air out. So I cannot see why you have boxed it in, especially as it is out of sight in the loft. This is not like an open vent pipe where you can get foul smells. I have a durgo valve in my loft and I have never experienced any unpleasant smells up there.
So yes, drill holes or remove the boxing, but I am not sure this will solve the noise problems.
THE AIR ADMITANCE VALVE ONLY LETS AIR IN TO HELP WITH FLUSHING.
IT WILL NOT LET SMELLS OUT, THE VALVE MUST BE ABLE TO DRAW AIR IN TO WORK PROPERLY, SO BE IN A VENTILATED SPACE. THERE MUST ALSO BE OTHER VENTED STACKS OF THE MAIN DRAIN IN YOUR HOUSE BLOCK[/b]