Our ears are very sensitive. The eardrum reacts to very small movements in the air, which we cannot feel on our skin but the ear picks up. Most people can hear their heartbeats when they go into a very quiet room. Because the ear is so sensitive, it is able to pick up the pulse from our bodies. If you hear a loud pulse all the time, you should go see your doctor. Plus it could be pulsate tinnitus.
It could just be a drastic fluctuation of blood pressure b/c the inside of our ears are sensitive and i do believe that the veins are close to the skins surface. As long as there is no pain you should be fine.
My ear sometimes does that too!! Nothing bad is going to happen