It is called pulsatile tinnitus and it does not have to be anything dangerous but it should be checked if it persists.
Common "dangerous" causes are arteriovenous malformations and intracranial hypertension. The first thing is checked by the doc listening with a stethoscope behind your ear and the other by the doc looking into the back of your eyes for signs of swelling.
In other words I recommend a visit to a doctor.
Take care!
Have you had a cold or maybe have a sinus infection? Congestion can cause problems with the eustachian tube in your ear and cause the problems you are describing. You might want to get your doctor to check your ears.