Water sloshing under dashboard

In mid-December 2008 my 2004 Buick LeSabre started making a “water sloshing around” noise under the dashboard. Having recently moved I took the car to a local shop. After 1 & 1/2 they were unable to determine the cause of the noise after checking the A/C drain, pressure testing and power flushing the system. I was asked to bring the car back when I had more time so they could continue checking for the cause. Fast forward to March 25, 2008 when the heater core burst (or gave out) dumping hot water onto the passenger compartment (and onto my right foot which was in a cast!). I then had my overheating Buick towed across town to my old, established, proven, reliable and trusted garage where they replaced the heater core and power flushed the system again. Alas, after driving away the water was still sloshing around ( sounds like about a gallon of water!). I then took the car back and they replaced the restrictor and said they had fixed the problem. We went home, parked the car until two days later. Upon driving out of the garage the water is still sloshing around. The noise is more pronounced when the car is first moved and continues for a block or two and then seems to subside. I have great faith in my trusted shop but it appears this problem is beyond their experience and expertise. HELP!!!

Check the area under the vents in front of the windscreen. The drains there can get blocked.

thanks for the advice, have looked under hood for the drains but unable to locate, will pass the advice to my mechanic.

There could be an air pocket in the cooling system, which would produce the sound you describe. This is not unusual after a heater core replacement. Some vehicles have a bleed valve at a point higher than the radiator cap to facilitate removal of air from the cooling system.

Again thanks for the advice. Of concern is that the water sloshing around behind the dashboard could be heard for weeks before the heater core gave out and leaked. My mechanic replaced the heater core and we could still hear the noise. He then inserted a restrictor in the line into the heater core and the problem still exists. I spoke with him and he believes that he will have to insert a restrictor on the hose on the outlet side of the core. Haven’t had a chance to take the car back as I am in a cast (can’t drive) so it may be late this week before I can get the car back to him with fingers crossed. I will pass on the comments you guys have sent.

Water that sloshes while cornering is usually in the ventilation system. The sound of sloshing water when you start the engine is usually caused by something in the cooling system/heater core.