Why do I hear sloshing noises in my car door after rain?
Quick answer: Sloshing from a car door after rain almost always means the drain holes at the bottom of the door are blocked. A little rain always gets past the outer weatherstrip and is supposed to run straight back out. Pine needles, road grit and old glass shards clog the holes, so the water stays inside and sloshes as you drive. Clear the holes; for a proper job, drop the door card, clean the cavity and reseal or replace the door membrane. Water reaching the cabin means the membrane has already failed.
That swilling, side-to-side noise in a door is almost never a mystery leak. It is trapped water that should have drained away and cannot.
Why water gets into the door in the first place
Car doors are not sealed. The weather striping that runs between the door panel and the glass wipes most of the water off the window as it drops, but a small amount always runs down the inside of the glass and pools in the bottom of the door shell. That is by design. The door has holes in the lower edge so the water can drop clear of the sill.
When those holes are open, you never hear anything. When they block, every shower fills the door a little more, and within a few weeks there is enough water inside to slosh audibly when you turn, brake or set off.
What blocks the drain holes
The usual suspects are pine needles and leaf litter if the car lives under trees, and road grit and fine silt that washes down the window and builds up at the bottom of the door. After a smashed side window, small shards of glass are easily missed at the very bottom -- they sit there and block the drains the next time it rains.
The quick fix and the proper fix
Poking a stiff wire up each drain hole from underneath will usually restart the flow. You will often see a dribble of dirty water come out straight away. That gets you out of trouble, but it does not remove the debris that caused the blockage, and the holes tend to block again.
The proper fix is to take the door card off, peel back or replace the door membrane, clean the cavity out properly, then reseal or refit the membrane. On older cars we usually recommend doing all four doors at once while the trim is off, because the rubber parts in all four were fitted at the same time and tend to fail together.
This Dodge Ram 1500 V8 had water getting through the front door area -- specifically along the rubber door tubing, which had deteriorated over time. The owner brought it to our workshop in Essex all the way from Cornwall. A common failure mode on larger American imports once the original door seals age, and one the owner had been hearing as a slosh from both front doors.
When water reaches the cabin
If water is getting into the passenger compartment from the door, the drain holes are not the main problem any more. Water has passed the door membrane, and that membrane has failed or been disturbed. Common causes are age, a previous speaker install that cut through the plastic, or a membrane that was not resealed properly after a window regulator repair. The fix in that case is a new membrane, correctly sealed to the inner door skin.
Sloshing you can hear but the door is dry
Not every sloshing noise comes from a door. The scuttle is the plastic trough that sits under the base of the windscreen and holds the pollen filter. It has at least two drains, one at each end, which carry water out under the wheel arches via drainage pipes. If leaf-fall blocks either drain, the scuttle fills with water and you hear it wash from side to side around corners. Left long enough, it overflows into the pollen filter housing and ends up in the footwells.
In both cases, reaching for silicone sealant is not a fix -- it seals off a drainage outlet rather than addressing a failed seal, and makes diagnosis harder. Why silicone fails on car leaks is covered in its own article. For the full detection process -- what equipment we use and in what order -- see what does car water leak detection involve?. For a broader guide to the entry points that let water into a car, see what are the most common places water gets into a car?. For the secondary damage that develops when a leak runs unaddressed, see what damage does a car water leak cause if left untreated?