Auditory Hallucinations; Cause & Treatment / Unclear?

In the past two weeks I have been hearing sounds and voices in a low volume that sound like it comes from a distance but there is nothing there. Examples of sounds and voices are: alarms, bells, horns, moans, whispers, gasps. My Dr. ordered a full Pet & Mri scan, blood work, however, nothing was found. I never had a history of health issues either. From what I was told by my doctor and reading about it, causes and treatments are highly controversial as nobody really knows for sure. Is this really the case?

Has anyone experienced anything like this?
Does anyone have any other solutions?