My HVAC system is forced hot air, with natural gas. The flue vent is making a weird noise when it's on, kind of like water sloshing around.
It's hard to describe the noise, so here's a video (it's a bit louder in real life than it appears in the video):
What could this be--what should I be checking?
Edit 2: I pushed the tubing upwards, so that the water would flow back into the furnace and the drainage tubing. Unfortunately, this has caused the furnace to stop heating. I don't see any flame when the thermostat calls for heat. Air is ventilated, but not heated.
Here's a picture of the furnace and the flue vent. The flue vent part that is inside the furnace is on the top left (black). In its connection, there are two drain tubes, that I think feed a left and right drain respectively. My furnace has its drain on the right, so would expect the left tube to lead nowhere (I can't really see or feel where it goes). The right tube leads to a little black box in the back, which has a connection to the external drain piping (which goes to a chamber with condensate neutralizer, and then a pump to pump it out into the sump pump hole). That all seems fine.
So what could've caused the furnace to stop heating? What if water came down the flue vent and it didn't all go into the drain tubes, but some of it went into the other connection? That seems to be the motor, right? But why would that have an effect on the flame/heating?
Edit 3: what caused the furnace to stop heating is that the inducer motor was flooded. When the thermostat called for heat, the motor would turn on, but it would have to move a bunch of water around, so it drew more current. Eventually the control board would kill it and output error E294 (on my Lennox) for "Combustion air inducer motor amp draw too high". The solution was to take out the motor and drain it.
I will also elevate the flue vent pipes to prevent the low point from pooling more water in the future. I also saw that the condensate traps need to be primed with water, to prevent gases from escaping that way. I didn't quite understand the mechanism, but I'll probably be doing that as well.