My Lumia 928 with 8.1 is doing the same thing today, drained the battery from full to nill in a couple hours and I didn't notice until someone called me and the voice was cutting in and out and in and out like clockwork every second also the screen turning on and off at the same time even though the phone was up to my face and the sensor should have kept the screen off. Some sort of popping sound in the rear phone speaker every two seconds. Go to settings -> audio and see if that setting page is freaking out cuz mine is. Its like the audio driver is crashing HARD every two seconds.
Its actually ok if I plug in a headphone cord, the noises and screen freaking out quits, as soon as you unplug the headphone cable it goes nuts again.
After plugging and unplugging the headphones several times the noise and screen freaking out has stopped... Something really weird going on and I don't like it!!!