A problem with a vent or the water heater will affect all of the drains in your house. If a sour smell comes from only one drain, the problem probably is isolated to the plumbing in that room.
The diagnosis: Your sink’s P-trap is clogged. The pipe under that smelly sink drain could be obstructed. The bend in the pipe—called a P-trap—can collect hair, toothpaste, gel, dirt and other gunk that will reek if it sits in there long enough.
The solution: Remove the P-trap and clean it out. To dissolve the clog without removing the trap, pour an enzyme-based cleaner like Bio-Clean down the drain every day until the enzymes can eat away at the organic waste that’s trapped in there. Then, treat the drains regularly to prevent future clogs.