There is most likely a blockage in the sewer piping, causing some air to flow back towards the air admittance valve. In this situation, the seal is pressed tighter against the valve. You can verify a blockage using the following test: 1. Remove the air valve from the end of the sewer pipe and place a piece of paper on top of the pipe. 2. Use a connected plumbing fixture; if the piece of paper flies off, there is likely a blockage.