"Can a blockage in the system cause this ? Thought a AAv / Durgo would open no matter what when a toilet is flushed etc , "
I would say blockage too, Air admittance valves let air in when its under vacuum , i.e if your sucking on the waste pipe it would open and draw air in from it instead of pulling the water out of the trap , in your case when the water is entering the system the air has to go some where because its blocked and that cant go out the AAV as it doesn't flow that way {otherwise smells would be entering the room} , removing the AAV allows the air to escape which is normal in this scenario , if the waste pipe was clear the air would escape naturally further along to the nearest vent..
I would say blockage too, Air admittance valves let air in when its under vacuum , i.e if your sucking on the waste pipe it would open and draw air in from it instead of pulling the water out of the trap , in your case when the water is entering the system the air has to go some where because its blocked and that cant go out the AAV as it doesn't flow that way {otherwise smells would be entering the room} , removing the AAV allows the air to escape which is normal in this scenario , if the waste pipe was clear the air would escape naturally further along to the nearest vent..
