I have a 2012 MDX Tech PKG with just shy of 150k miles on it. 2 nights ago I noticed a high-pitch ringing sound coming from what sounds like the back of the vehicle. It literally sounds like tinnitus (ringing in the ear). The sound is first audible when the key is turned to accessory mode, so I'm thinking something electrical. The sound does continue when the vehicle is started and I'm driving about. I've played with the AC and audio settings, but nothing kills it. Until recently I always had the sub turned down with the bass and treble at their default settings. Lately I've had the sub turned up along with the bass, and the audio diverted slightly to the rear of the vehicle. If one of the rear speakers goes bad, would it make this sound? Could it be a wiring issue with the factory sub? Just want to get an opinion to see if I can fix it before I take it somewhere and likely have to fork over some cash.