This can be tinnitus, and there can be many causes. It is best to see a doctor for this, particularly an ear nose and throat specialist, if it persists.
Possible causes/solutions if it is temporary:
Crackling caused by pressure ~ this can be stopped by swallowing-drink some water or eat a lollipop.
Crackling caused by water in the ear ~ sharply jerk your head to the side of whichever ear it is or poke around in there with a tissue or a cotton bud.
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Well first off, if your ear doesn't crackle. It's more of a ringing sound, like if someone slapped you in the ear. If you are experiancing a "crackling sound" as you described, then you should definitely see an Otolaryngologist (Someone who specializes in Ear, Throat, and Nose issues). But ringing is fully normal, it's just basically when you have taken an overdose of decibels (a measurement of sound). And that's just occasional thing that every single person has experianced.
it could be that you never cleaned your ears in a while. go clean your ears.
or it could be that you went to go swimming and got water in your ears.
Also , could be the natural thing that happens when , if you drink it can help.
Ear-wax ! A bulid-up of ear-wax causes tiny 'popping' sounds as the crystals move around the ear canal. Treatment is either by medicated ear drops (any pharmacy will have them) OR - by syringing (done by a nurse or doctor).
Sometimes i hear my heartbeat through my ear drums. I dont know if this is normal or not, but it could be the cause of the pulse/drum sound your hearing.