OK so I was worried about my coolant being very black brown color. So I did a lot of tests one was taking off the rad cap and looking for bubbles. There was none, so today after driving a little the car was on normal temperature when I got home I wanted to check something, so I left the car on popped the hood and unscrewed my reservoir cap, while idling I noticed some bubbles coming from under one corner of the reservoir, is this normal or does it indicate a head gasket leak? The reservoir doesn't boil the water just has some bubble's