I can voluntarily make thundering sort of noise in my ear. Is this normal?

Can happen. Some people can flex the muscles in and around the ear and make sounds that they can hear. It is not typical but can happen. If you are also experiencing any changes in your hearing you should get checked.
Not common, but OK. But most likely you are voluntarily contracting your stapedius muscle in your ear. This tiny muscle contracting can cause your ear drum to tremor, which might sound like thunder or a drum beating. F it is voluntary, it is almost certainly not harmful, but you may want to have it checked out by your local ent. Best wishes.