Hi,you are suffering from a condition called
Tinnitus.It might occur after an
Ear infection.It is not a life-threatening condition.But it might cause irritation as you are feeling.
Eustachian tube blockage(a tube which connect ear and nose and responsible for equalization of ear pressure with atmospheric pressure) might be a reason for the hissing sounds you are hearing.
You can try
Valsalva maneuvre (done by closing one's mouth, pinching one's nose shut while pressing out as if blowing up a balloon).This will help to open the Eustachian tube.
Also,you can take decongestant nasal drops like
otrivin nasal drops,Anti-histaminic like
Cetrizine and a multivitiamin tablet.
You may continue with Betahisitne.
Avoid hearing loud sounds.
If you are familiar with pranayam,Brahmri pranayam also helps in tinnitus.
Do not pay too much attention to the hissing sound,try to ignore it.
You can hear White music(available on net like youtube),particularly in the night ,when the hissing sound is annoying.White music has a masking effect on Tinnitus.
If you are also suffering reduced hearing along with the hissing sound,then it is advisable to do a hearing test named Pure Tone Audiogram.
If still your hearing of hissing sound continues after 1 month,it would be advisable to again take follow-up of an
ENT Specialist.
Hope this answers your question.Take care.