Why does it make this “watery” noise?

there is oil inside the fork, primarily used for lubrication so that the fork cycles smoothly.

These have a small amount of oil in the fork lowers for lubrication — from the factory it will be Fox 20 wt gold in the air spring bath and 5 wt with Teflon in the damper bath, which cycles through the Grip damper circuits. The 20 wt doesn’t really slosh like this, the 5 wt might but this sounds a little less viscous than I would expect. It *could* be some water has gotten in but that would be a lot of water.

Regardless you are almost certainly due for a lowers service. They’re easy enough to do with home tools and a seal kit, but the oils are sort of an investment. The 5 wt Teflon has been phased out in favor of 4 wt without Teflon (presumably for environmental reasons).

Pull out the seat and attached post