It probably has nothing to do with the lipitor.
What you are hearing is just the blood rushing by the middle ear -- because the main blood vessel to the brain, the carotid artery, passes in the bone right next to the middle ear, and you can sometimes hear your pulse this way.
If it starts occurring continuously, it is called pulsitile tinnitus, and further investigation would be done to make sure there wasn't a specific issue causing the problem. But because it seems to be random, it may be just as simple as some earwax in the ear, causing you to ear more of this, or just a few episodes of a stronger heartbeat, or something else random.
But a lot of people have random episodes like this. If it occurs more frequently or continuously, then discuss it with your doctor.