I am going to have to disagree with your analysis. A noisy pipe doesnt equate to a blocked pipe. The communal waste system is by its very design going to produce a degree of noise through use from the flats above your own.
If there were water and indeed smell then the problem would warrent investigation by the managing agent. If you really are that worried by noisey pipes then I too would recommend you get a plumber in to listen and give you thier diagnoses.
All the best
I have just noted you are on the second floor so I suggest you consult your neighbours below and see if they share your concerns. If your collective analysis is the same, then I am sure the managing agent would investigate on that basis.