I have bathroom vanity with two sinks. With its drain plug open, the left bathroom sink fills. If I turn off the faucet and let it drain (goes very slow) then start the water again, it's OK and will drain normally. The right hand sink seems unaffected. Does this mean the roof vent for this drain line is plugged? I'm guessing that long Ponderosa pine needles may be the culprit. Do I need a plumber to clear it or can I snake it out?