> This is the same reason you “hear the ocean” when you put a shell to your ear.

This is incorrect. The noise heard from a shell or cup is ambient sound echoing around inside the container, not blood flow. You can prove this yourself by taking a cup and pushing it against your head so it seals around the edges. The rushing sound will greatly diminish (some will still leak through or be vibrating the cup itself), while of course blood continues to flow in your head.