This could also be a very simple issue of the 3.5mm plug not being inserted properly. The headphone jack of Zenfone 6 requires more than usual amount of pressure to insert the plug properly. I faced the same issue before realizing it wasn't really an issue. If you have a thick case on, this condition becomes even more stealthy. You could just try pushing a little harder and the problem might be solved (Pun not intended ?)
Are there any audio outputs that would be making noise? For example a live call, or a video? Does the sound seem to be static or some kind of electrical interference?