I have reviewed the previous answers to this problem regarding a blocked vent. However, I have 2 toilets and 1 vent. 1 toilet is fine the other drains. Also,in the bathroom with the problem, the sink gurgles when you drain it. I have,on three occasions corrected the problem by shooting water from my water hose down the vent pipe but days later the problem returns. Any ideas? Thanks in advance for your response