It just started today. But about every 30 minutes or so, I can here it gurgling. No one else in the house is running water, flushing a toilet or whatever when this happens. What hell is that?
You have a blockage somewhere in one of the lines. It isn't backing up where it is visible, because you aren't using enough water. As it works past the blockage you hear the gurgling.
I believe that is your AC drain. Our primary is via the sink upstairs. If that stops, then our secondary is an outside drip that we have. If I start seeing stuff dripping off, that is bad news. That means that our primary is clogged. And need to get it looked at. I am no plumber but you might want to find out where your AC stuff is.
At our house that noise signifies that the main wastewater line is clogged. The sound usually first occurs in the toilets at our house, but you can hear it at pretty much any plumbing fixture. There's tree root incursions and I occasionally have to take care of it with copper sulfate for the roots and plunging the main drain line. If I was you, I'd run the shower/tub and then go check your drain line (assuming there's an access pipe outside your house).
it's probably the a/c (black hose tied in to your plumbing under that sink). that line might not be draining properly (black hose sagging too much?) or it's syphoning air....
I agree with The Fied that you need to check your secondary A/C drain line and look to see if there is any freezing at your A/C coil. I would also buy some Thrift at a plumbing store to unclog the drain in case that is the problem (don't use Drano). In fact, a little Thrift down the sinks and showers every 2-3 months helps keep the pipes clean.
