The presence of sewer gas (rotten egg or methane smell) in the home is a critical red flag. This indicates that a trap seal has been siphoned dry due to vent blockage. This is a hazardous condition requiring immediate attention.
When a sink drains slowly, the immediate assumption is a clog in the P-trap or the branch drain line. However, if clearing the P-trap does not resolve the issue, and the drain still drains slowly despite no physical blockage in the waste line, the culprit is often a lack of vent air. The water is being held back by the vacuum pressure, much like a finger holding water in a drinking straw. clogged vent pipe