Recently I started getting s slow drain on a double kitchen sink and the water would well up in the empty part of the double sink. At the same time I started getting sewer gas smells in the sinks when they were sitting empty. I could eliminate this smell by letting water stand in the sinks with the plugs in place. The drain pipes themselves do not seem to be plugged. I am wondering if the vent for that stack is plugged from the falling needles of the numerous evergreens overhanging the house. If that is the case how would I best clean out the vent which is accessible only from the top end of the vent? Any suggestions/