I had been annoyed by a strange noise in my study room. It sounded like someone hit a wall, the volume is small, but it made me frustrating. I noticed the sound about three months ago. At first, I thought that the sound came from the neighbors in the apartment, but it was wrong. I found it sounded only when the fan in the bathroom run.
I checked the fan but couldn't find what's wrong. I call the support center of the management company for the apartment this afternoon, and they promise to call an expert to investigate the problem. I was happy to hear that. Just after the thought, I came up with a new hypothesis of the reason. I carefully watched the bowl hanging from the bathroom wall, and I found it was moving a little bit, hitting the wall, and making noise. I put it down, and then the noise stopped. It was the true reason for the strange noise.
I was delighted to solve the problem and to stop the annoying noise.
