Popping / Cracking noise when I add coolant...gulp

[Arlo]

Hello all,
Yesterday on i_95 my coolant return line blew. I pulled over as soon as i noticed the high temp gauge but i'm guessing I was too late. Had it towed home, fixed the hose, started to add coolant and these i heard these strange popping noises.

I'm fearing for the worse here.

[ArmedAviator]

Sounds almost like plastic being hit by something. Check your radiator fan blades and belt to ensure they are not touching/rubbing anything.

[Arlo]

nope nothing rubbing

[EdBwoy]

I was going to guess that the pump was cavitating, but that is usually a more consistent sound. On the large industrial pumps I work with, cavitation sounds like you're pumping gravel.
Other guesses:
-Bad bearings on the fan or the pulleys.
-Are your belts still in good shape - good alignment, good pulleys, no cuts or frays on the belts?

Can you pinpoint where the noise is coming from - stethoscope or good solid rod prodding around the engine bay?

[Arlo]

I think it's coming from the coolant. With the cap on i don't hear anything.

[EdBwoy]

What part of the coolant system specifically?
And that sort of makes sense, when you remove the cap you significantly drop the pressure in the system and then the coolant will boil at a lower temp/boil easier. From where I'm seated it doesn't sound like boiling sounds, would you describe it as such from your perspective?

[ArmedAviator]

I think it's coming from the coolant. With the cap on i don't hear anything.

Well if it's filled up than install your radiator cap and drive it like you stole it. Sounds like case closed to me.

[Arlo]

I wish i could. The temp goes right back up to H after a few minutes.

[ArmedAviator]

Are both upper and lower radiator hoses getting hot? Is your heater blowing hot air?

[Arlo]

The upper hose gets hot the lower does not. Yes it blows hot air.

[ArmedAviator]

Replace your thermostat - seems like it's not opening.

[Arlo]

You sir are correct. Thermostat replaced and temperature holding steady. Thanks all