Can you hear/feel that noise, I asked R. Yes. It was something between noise and vibration throughout our apartment. This was when I was home for lunch. When I arrived home in the evening, it was still happening and seemed a bit worse. We managed to sleep in spite of the noise.
I peered over the balcony and I have never noticed the air con being on at the apartment below us. That is not to say it hasn't been on. I just had not noticed it being on. It did sound a bit noisy. I remarked about a strange noise on the balcony a few days ago to R.
When I came home the following day, I checked a few other floors and the noise seemed to be going right through the building and it was even worse. I called building manager who told me it was under investigation. When I arrived home that evening, the noise had stopped.
It was few days before I ran into the building manager to ask him what the noise was. I was correct, the air con unit below us. The owner had gone to Malaysia for three months and left the air con. Clearly something was wrong with it, bearings by the sound of it, hence the noise. The manager had the phone number of a friend of the person away and he rang her and she told him it was ok to turn the power off in the service room, which he did and the problem was fixed. He told me he had calls from residents from the 9th to the 18th floor about the matter, so you can imagine how bad it was.
It is quite weird that while we don't hear noises from other apartments, a simple faulty air con unit outside on a balcony affected over 100 people.