A couple weeks ago my sinks, toilet and tub were gurgling and draining very slowly. I called a plumber but by the time he came, the gurgling and slow drain had stopped. He said all the drain lines were clear. He even snaked out the line from the house to the backyard and said there wasn't any obstruction.

Today the problem started again. When the tub is draining, there is loud gurgling in the sinks and bubbles coming from the toilet. The drains are all slow and the toilet even overflowed once. The plumber never mentioned the vent pipe at the top of the house but from what I've looked at online, I'm wondering if that could be it. How do I check? I can't really afford the plumber again. :(