About six months ago I started hearing a loud gurgling noise emanating from my kitchen sink when I was not using any appliances. I initially assumed this was due to people living in the apartments above me using new appliances (such as a power shower) and that there was not anything that I could do about it.
However, a plumber told me that the gurgling noise could be due to a "partial blockage in the stack". He told me that a faulty "water seal trap" can be identified by either a gurgling noise and/or a foul smell.
I informed the buildings owners of the problem and they were skeptical. They told me that they would pour something down the drains to clear them. But this has done nothing to resolve the problem.
Is this loud gurgling noise more likely to be caused by people above me using more water than before, or a faulty water seal trap. How can I convince the buildings owners to investigate this possibility?