Customer had us out today to inspect a gurgling sink. We found the cause to be a clogged ac condesate line as the condesate line tied into the drainage for the sink. We installed a blow out valve on the condesate line and blew the line clear. We then tested drainage on the sink and everything was working great. Gurgling noise persisted. Talked with master plumber and ac tech that was here and we agreed that another vent stack needed to be installed on the ac condesate line to eradicate issue.