hammerhead+3

Just bought a 2018 Sierra Denali. Drove up from Miami to Titusville no problems. Started driving the next day and an intermittent vibration appeared out of nowhere. Thought it might have been the road I was travelling on but it persisted. Engine idle goes up a little while in cruise and then the vibration occurs for a couple seconds then stops. This occurs at about 30 second to one minute intervals. At first I thought it was only at highway speeds, but I have noticed it as low as 45 mph. Does anybody have an idea what might be going on?