you better hope it's the cap and get one from toyota not a parts store
you might pay 7-8 bucks more from toyota but spring for it .
the water rushing sound in the morning is from the lack of the proper water level
get the new cap , top off the coolant , i would park the car on a serious incline to try and get any and all air out of the coolant system .
if you lose more water and hear the rush again you have a BHG
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