As surprising as it may sound, it is the muscles that make noise during their contractions. When you close your eyes, the muscles tense (contract) and make a very low sound, this is the very hum
If this humming is similar to the hum when yawning or swallowing, then this sound can be caused by the passage of air currents through the open Eustachian tube, which leads to the oscillation of the eardrum.
It opens when it is tightly closed due to contraction muscles that lift the palate and are needed to equalize pressure in the inner ear and the environment.
Is the buzzing pulsing to the beat of the heart? Most likely, this effect is caused by the squeezing of the vessels by the muscles, due to which the movement of blood from the laminar becomes turbulent. Vortices in the bloodstream generate noise. Also, noise can be created by the muscle fibers themselves, chaotically contracting in a tense muscle.