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.
That happens to me too. It's called Tensor Tympani Syndrome (TTTS), which is probably a result of Hyperacusis. There are subreddits for those too. The most important thing is not to overprotect your hearing since that will only make your ear muscles even more sensitive. Feel free to message me if you're having trouble :) My symptoms used to be quite bad but nowadays I go weeks without thinking about it, so I'm practically cured.
Elon Musk and Mark Zuckerberg agree to hold cage fight (www.bbc.com)
Earlier this month, Meta showed staff plans for a text-based social network designed to compete with Twitter.
Thunder in your head? (kbin.social)
A long while back I was hanging out with one of my sisters and she said that she hears thunder in her head when she gets startled....