Water sloshing noise behind speedometer

You may have a problem with the fresh air box not draining rainwater. There are drain or weep holes at each end of the fresh air box. they can get clogged with debris after years especially if you park outside under a tree. You can check this by removing the inner fender well inner liner so you can check the weep holes as they should be clear and if you pour water in the air box it will come out of them.
The other possibility is air in the cooling system. Follow the link below to get instructins on bleeding a cooling system.

https://www.2carpros.com/articles/vehicle-bubbling-noise