[DQDick]

If you do all of the above look under the sinks in both areas and see if they haven't put an inline vent on the drain line instead of running a vent thru the roof. Islands for sure have them and so do some bathroom sinks. They look like a plastic can on the end of a drain line. They are spring loaded to let air into the line and can (particularly the cheap ones) become stuck open in which case they can vent air directly from the tank into the under sink area.