I've been keeping a mental track at the moment, rather than detailed records and have identified what I think are a couple of properties which seem to match the pattern as to when it is and isn't caused. There was a period in the summer when the noise stopped for a period but the management company weren't extremely helpful with enquiries with other properties.
As for the pipe not being properly fixed, should I see movement on it? There's none that I can see. The mains pipe the sound originates from comes into my flat from the below floor, and goes a few feet directly to the sink (with a stop cock and joins to my boiler and bathroom en route. Also the sound is the same when it is heard under my sink and at the point of my stop cock when I close it and it moves there. That then leaves next to nothing of pipe in my actual property.
