recently got an all-metal radiator. today after driving I checked the coolant. after letting the car sit for about an hour or maybe a bit less, I loosened the cap and I could hear a hiss as if pressure were being release from the expansion tank. is this normal? when I screwed it back on, I made sure it was snug and tight. should I do that? also, do I need a new radiator expansion cap? black? thanks.