We bought a 1967 year old ranch 2 years ago and have been experiencing a foul smell coming from our bathroom sink. We have been putting bleach, vinegar and soda down the sink and overflow over and over again but the foul smell keeps coming back. What do you think the problem may be?
A blocked vent is probably the most likely culprit. Other things to consider: leaking plumbing from a low point in the trap under the sink letting the trap drain, evaporation of the water in the trap if this sink is rarely used.
Some sinks do not have a true vent, and may have in place aa AAV (Air admittance valves-AAVs or Durgo valves or Studor vents). In my experience, these are poorly designed and allow sewer gas to seep through.
Sometimes really old hair that has been in a drain pipe for a long time will smell pretty bad. Not a sulfur smell like sewer gas, but more like a poop smell.