Recently our 2006 v50 has made a really noticeable sloshing noise apparently coming from somewhere in the engine compartment, but it ONLY happens the first time I brake AFTER the car has been sitting still for a long time, like overnight or at the end of the workday. And then it doesn't happen again. I've read all the threads about blocked sunroof drains and A/C and water in doors, etc., and I don't think it's any of those because it only happens once a day (usually) and only when I brake for the first time, not when I accelerate or turn. The local Volvo mechanic--excellent guy!--can't look into it because by the time I get the car to the shop it's not making the noise any more. Can't fix a problem that's not there! Very curious.

 

My bet is that it IS due to a clogged A/C drain. More specifically, due to debris blocking the drain from the airbox.

I'd do a simple test to find out if that's the case - just drive carefully until you are somewhere you can stab the brakes. If you get the slosh, stop and look under the car and see if you can see an obvious stream / trickle of water. If not, try the brakes again and see if you still get the "slosh" (is that a noun?). Repeat until it stops and/or you get drainage.

I can imagine a scenario where the sloshing of the water in the airbox is enough to barely dislodge some debris around the drain hole, after which it re-blocks the hole and "firms up".