Pour down some Coca Cola and leave overnight. You may find it solves the problem for you. Probably build up of hair in the trap!
Thanks I will try that.
Make sure the P trap is holding water. Pour a cup of water down the drain. If the odor goes away then something is sucking the water out of the trap. Turn the faucet on and see if there is movement in the toilet water level. Flush the toilet and check if the if the odor comes back. If it does then the problem may be the roof vent. Can be blocked.
Could also be a dirty overflow hole in the sink. People clean the sink drain but don't think about the overflow hole.
Thanks William, I will try that this weekend.
Hello there. Cindy here. Begin by pouring a quarter cup of baking soda in the drain. Follow with one cup of white vinegar. Do NOT turn the water on. Let it sit for two hours with the bathroom door closed, After that slowly pour a gallon of very hot water down the drain. Wait 15 minutes then run cold water for about 15 minutes. That's it. Problem should be solved. Wishing you all the best.
William pretty much said everything that I was thinking.