Many thanks for these pictures, but can you post a bigger picture of the whole engine bay, and mark where exactly to drill the holes on each side? My L322 is a tdv6 version from 2003.

Yep, will do
Give me a day or two Rob

Thanks mate, it's driving me crazy. I take it you have also tried other alternatives like venting the coolant fluid with the heater on etc and I didn't work?

I hope these photos help.
I only had occasional problems because 99% of the time it is parked in a shed.
But when I went away, for say a week or two, and it was parked outside, I would get the sloshing if there were consecutive nights of heavy overnight rain.
Back home in the shed, it would gradually die away, I assume through evaporation.
Anyway since doing this the problem has never come back.
And the slight rust around the hole I drilled a few years ago maybe suggests that a bit of moisture passes through it at times.

photo 1 - long shot of engine bay (V8 BMW here)
Photo 2 - stuff removed to allow heater air intake filter housing to be removed
photo 3 - heater air intake filter housing now removed (and sitting on top of radiator)
photo 4 - zooming in on the 6mm hole I drilled a few years ago
Photo 5 - close up of same

Thanks a lot, these pictures really help. I will take some time and drill those holes.

I also suspect something might be wrong with the heater matrix in the cabin as I ocasionally see some drops of water on the passenger side carpet and I am suspecting a minimal coolant leak. But this is probably not related to the water sloshing, unless it's dripping down those two big holes into the cabin?