You have two bathrooms in a mobile home.
The front toilet flushes with no noise.
The back toilet flush causes gurgling under the living room floor.
You cleared both roof vents and found no blockages.
The washer does not back up into tubs.
Everything seems to drain fine.
So why the gurgle?
Quick Q&A
Q: What causes gurgling under the floor when only one toilet flushes?
A: It is usually air getting pulled through a nearby trap because that branch drain is partly blocked or poorly vented.
What the sound is telling you
Gurgling is air moving through water in a pipe.
That often happens when a drain cannot “breathe.”
A fast toilet flush pushes a big slug of water.
If air cannot enter behind it, pressure changes form.
That pressure can pull air through a trap.
The noise may travel under the floor.
Most common causes in this setup
- Partial clog in the back branch line. Flow still works, but air fights for space.
- Vent issue on the back group. The roof vent may be clear, but the branch vent path may not be.
- Loose or sagging belly drain. Mobile homes can develop low spots that hold water.
- Trap siphon risk. Pressure pulls water from a nearby trap and makes noise.
Simple checks you can do today
Start with the easy, low-risk items.
- Fill the back tub and sinks, then flush the back toilet.
- Watch for water movement in those drains.
- Listen near the back bath wall and floor registers.
- Smell for sewer gas after flushing.
- Check the toilet bowl level after a flush.
If the bowl level drops, a trap may be losing water.
That points to venting or pressure problems.
Target the back branch drain
A “no backup” drain can still be partly blocked.
Mobile home lines can collect paper in fittings.
Hair and soap can build up in a shared branch.
Try a proper closet auger on the back toilet first.
It clears the trap and first bend safely.
If the gurgle remains, the clog is likely farther out.
When to suspect a belly drain issue
Many mobile homes use a covered belly space.
If a drain strap breaks, the pipe can sag.
That sag holds water and slows air flow.
You may still get “good drainage” most days.
But the flush surge triggers gurgling.
When to call a pro
If gurgling gets worse, act fast.
Gurgling can come before a real backup.
A plumber can camera the line and check slope.
They can also test venting and trap seal loss.
Helpful external resources
- EPA overview of sewer gas and safety
- Home inspection guide to plumbing venting basics
- Toilet clog clearing steps and tools
One last tip.
If you smell gas, refill all traps with water.
Then book a service call soon.
I’m Chris Mayer, writing for Plumbing 101.