I have a service connected disability with respect to a phone, believe it or not, that was at my desk and rang with a high pitch so loud that it caused me pain. It took awhile before I noticed the damage it was doing and now I have a high pitched whine to listen to 24/7. It is only in my left ear (the side the phone was on). Unfortunately, the hearing tests could never pick it up, because the pitch of the ringing is apparently above what they test for. I have had the ringing since I was in my twenties. Most people think of industrial activities or loud music that causes damage to the ears, but causes are many.
With that said, let me tell you that I am 100% convinced that the only way your Volt is in any way related to your hearing issue is that it is so QUIET in the Volt, that the ringing becomes more apparent. Background noise always helps to mask the ringing and the Volt may have removed enough of that background noise that you are now keenly aware of the hearing problem.
2012 Volt (Red) 'SUN FUEL'/2012 Volt (White) 'OVR OIL'/2008 Tahoe Hybrid
Driving Without Destruction