I've recently noticed, when i'm idling, and i bleep the throttle, when the RPMs are dropping, i hear a watery noise? Like if water was being moved or something

What the hell can it be? I don't remember installing an aquarium in my engine bay...

 

I just noticed the same thing the other day in bumper to bumper traffic. I don't know what it is either, maybe something to do with the cooling sytem. Could be the slight increase then quick release in throttle is giving the water pump a quick little surge then the pressure releases making the overflow tank gurgle a little each time. Could be way off but that's my best guess.

 

same thing here.
i hear it when i start the car for the first time.

 

if your not the only one then it must be normal

 

I haven't heard it in my z but in some other cars it's air in the coolant. If it is air in the line you should see the temp tip up and then back down.