Sure sounds from how you have explained things to be most likely the expansion and contraction of waste water pipes following use of a handbasin. Once when they are heating up and once when they are cooling down.
The materials used are more than likely PVC hence the loud sound.
Insulation of the cavity providing the material is packed tightly round the waste pipe will assist in easing the problem for you.
Otherwise set your alarm and start mowing the lawn outside your neibours before he rises to get your own back!!! 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
