weird sound after 30minutes of being on

[Pastechi]

Hi guys, i'm new in the carworld and finally got myself my dream car. the R33

I bought a 1997 40anniversary R33 GT-S 3weeks ago. I gave it a service(change oil, oilfilter and sparkplugs) and the car runs great.
I don't drive much.

20minutes to work, and 20miuntes back home. There are times when the car is on for more than 30minutes(Incase i have to drive my gf somewhere before home) that my car sound turns into this gruggling noise but my meters seems like nothing is going on.

One thing that comes to mind that i am not using a coolant. Just using water atm in the radiator and i think that the water becomes so hot that it maybe causing the gruggling noise to happen.

I can get home, turn off the car and start it back up tomorrow and the sound is back to normal like nothing happend. just that if the car is on more than 30minutes i hear the gruggling noise(doesn't sound like a bubble noise, just a harley davidson idle ish overheat noise idk how to explain)

My car is currently at 394WHP if i'm on stage B(22psi) but its currently on stage A(16psi).
I tried hitting up 3 local mechanics but they told me it could be my thermostat or a blown headgasket but they seem too busy to give a hand. So i thought to ask on forums.

I thought maybe flushing the radiator untill the water end up coming out clean, and get an antifreeze coolant should do the trick but idk if the issue might continue.

Any help would be nice

[PranK]

Closed as dupe. Original is here;