Grinding noise coming from...coolant tank?????

[Sketch]

So, I bought a 1994 6.5 2500HD a week ago this tuesday. The noise started yesterday morning. I notices a random grinding noise coming from the passenger side. it just happens at random times. today I caught it with the hood up and it is coming from the coolant tank or just below it. almost sounds like a loud fast fart or gears grinding when you mis-clutch a manual transmission. My coolant level light is on but the coolant is topped off, I think that sensor is bad becasue temp hovers just below 210 like it should.. any ideas? I am stumped

 

[skylark]

Are you sure that you are not hearing air bubbles? Head gasket issues are common on the 6.5 and it will push air into the recovery tank.

 

[Sketch]

Ended up being the gasket on the coolant cap. was heavily dry rotted and wasn't sealing. the sound was the tank releasing pressure. probably why my engine was running a tad warmer than usual as well. Cut myself a new gasket, re topped of the coolant and problem solved. no low coolant light temps at normal and no wiered noises. level is also staying up where it should.