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The cause of tinnitus is usually idiopathic. Every normal person can hear that sound many times in their lives. It goes away as suddenly as it had started.
There is no reason for worrying as you have no hearing loss. So relax, do not worry as these kinds of tinnitus is intermittent.
Sometimes some situations, some kinds of food can start that tinnitus.
As you reported that the sound was on scale 3, please try to avoid it.
If the tinnitus is disturbing you, consult audiologist for TRT (tinnitus retraining therapy).
For more information consult an audiologist online --> https://www.icliniq.com/ask-a-doctor-online/audiologist