I believe this could be the result of the tensor tympani muscle activating, which it tends to do in fight-or-flight situations. It is located in your ear and has the purpose of lowering sounds that may otherwise make you deaf. I remember there was/is a subreddit called r/earrumblersassemble , consisting of people who claimed they could voluntarily "rumble" this muscle, and I happen to be one of them. Anyway, it does resemble a thundering noise, and it does activate when I'm startled.