On hot days and the air-conditioning is running there may be a dripping sound coming from under the sink area in one of the bathrooms. This sound may be the sound of condensate (water) dripping into the sinks P-trap area where there is standing water. The line that goes into the sinks tail piece drain is generally the primary condensate line from the A/C unit (often located in an attic area).
This noise only occurs sometimes when the A/C unit is running and may often be overcome by changing the angle of the pipe entering into the tail piece section.
Who to consult? Plumbers and qualified handymen.
Home owners can usually solve and fix the cause of dripping sounds under the sink themselves. Plumbers can easily handle this type of work, as well as many qualified handymen.