Yeah, its been very warm here also, temps in the 90's. I don't know about any "sound" coming from the overflow bottle, but as long as the fluid level is higher when hot than it is when its cold, the fluid returns to the same level every morning before you drive, and it doesn't blow out of the overflow bottle then you are good, thats how its suppost to operate.
This is what I do: I use a black sharpie and mark the level with a dot when the engine and coolant are at normal operating temperature. Then you can check it every time its stone cold or up to operating temperature. The main thing is to make sure it returns to the same level when its cold, like after it sits overnight.
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