Noise from engine bay after taking off radiator cap

[theasset]

This morning I was checking coolant level. Noticed it was a little low so took radiator cap off to check it was all good then a noise started in the engine bay. It sounds like a kind of hissing/light squealing noise.

It's still there when I drove around the block, came back and took cap off again, revved the engine and it seems to go away. Put the cap back on and it comes back.

I've never heard it before and it only started when I removed radiator cap, and it goes away when I remove cap again. Not immediately but after a few revs then returns when I replace cap.

 

[Old.Tony]

It might be air in the system. Find a nice steep bit of ground somewhere, park it nose-up and give it some more water. Turn the engine over for half a minute, turn off and do it again. Then see if it continues.

 

[theasset]

It might be air in the system. Find a nice steep bit of ground somewhere, park it nose-up and give it some more water. Turn the engine over for half a minute, turn off and do it again. Then see if it continues.

Thanks @Old.Tony , it ended up being coolant on one of the belts. Went away after a couple of days.

Not sure why I thought it was with cap on or off. Must have just been the timing of noises.