Sloshing noise from the rear?

[donald]

I've got a noise that sounds like a gallon of water sloshing around in the rear - wondering if any of you have experienced it or know what it could be.

I've got no rear seats or interior, and a half cage, so maybe it's extra easy for me to hear, but I've had the cage for many months now and this is the first time I've heard it. It's when the car stops or starts, but does not happen when the car is still.

Things I've looked at:
1. There's no water in the black plastic trunk well, or the battery/other side back there.
2. I've given both doors a good shake (it's an E92) to see if the water's there.
3. I've filled the gas tank, to double check it's not the fuel. Coolant level is also good.


What I haven't tried:
- Unclogging the evap tube under the car


Thoughts? Past experiences with this?

[Bmwdeven]

My car has a sunroof. When I first purchased it I wasn’t aware that the drain tubes were clogged from the sunroof. I only found out when I heard sloshing after a a few days of heavy rain. Turns out that water had breached from multiple points and soaked into the carpet. Took me multiple days of hanging the carpet and fans to get out 100% of the moister. Assuming you have a sunroof that could be the case with your car. Either way push your bare hand on your carpets to check for potential water.

[tbuck]

It is your fuel moving through the baffles in the tank. I hear this every time I back out of my driveway because the car is quiet at idle. It changes with different fuel levels but it is almost always there.

PS I do not have a sunroof.

[MineralWhiteF80]

this isnt it, but just to cover all bases, I'll tell you a story...

In my F82 M4, I've had the exhaust valves forced open 100% of the time, which bypasses the muffler chamber completely, for years. It started making a sloshing sound, so I recently took the muffler off and it was FULL of water because of condensation and whatever other ways water gets in. The muffler would just keep collecting water and never drain out. Of course I couldn't get the water all out easily, so I forced the exhaust valves to stay in the closed position and under full boost, you should have seen the massive amounts of steam and water clouds pouring out of the exhaust under full boost/full acceleration After a few good runs to redline, the muffler dried out.

[donald]

Thanks all. No sunroof or valved exhaust here.

Fuel tank - if so, I’m wondering why I didn’t hear it before and why now it’s so obvious. And wouldn’t filling the tank to the top mean that I shouldn’t hear anything?

also: it’s every single time the car starts or stops, not just backing out or once in a while.

[steven_s]

lol awesome because I just noticed this too for the last month. Gas moving through does make sense. It also usually happens just at the start.

[M3SQRD]

Slosh frequently, and how well it’s damped, changes with fill fraction (% of tank volume filled with fuel). It’s a highly nonlinear relationship between fill fraction and slosh behavior. However, the slosh effect doesn’t completely disappear with high fill fractions.

Besides the fuel tank (and apparently the muffler!), what else in the back of the car is capable of producing a sloshing sound? Why you haven’t heard this before? I have no idea.

[Helmsman]

You guys must have the Noisy Gas. No slush no never over here...

[--Aramis--]

Exhaust too quiet or drive too slow...no this kind of noise here also

[Ximian]

Spill something in the trunk? There are holes where it can go into for draining out of the car. Lift up the cover in the trunk and take a look around.

[donald]

A rainy track day + drive to/from later... I figured I'd check. I think I found the culprit:



In the far side of the battery, there's a dirty puddle. It's below the level of where the battery sits, and I guess I didn't look hard enough the first go-around. Gonna take the battery out and figure out how to drain it.

Seems like this has happened before (as other threads exist). Some ideas:
- doesn't seem to be from the taillight? The top of the battery is dry, unless there's another way it would get in from there.
- maybe the fuel door. I'll spray the outside and check to see if any's getting in.
- otherwise I'll fish around...

Anyways, I'll keep the thread up to date. Appreciate y'all.




1/26 update: Had a shop look at it actually. The leak was actually a hole near the bottom-side of the taillights, so the water wasn't contacting the top of the battery, but still seeping down from there. Added some seals to that area.