2005 Sonata making odd "grrr-ing" noise

I have a 2005 Hyundai Sonata, 6 cyl. with about 89,000 miles on it. In the last few days it has started making what I can only describe as an odd "grr-ing" or "errr-ing" sound, intermittently, exactly every 15 seconds, which only lasts for about 1-2 seconds. It seems to be coming from the engine compartment. There is no loss of power or stalling. It does it regardless of whether I am accelerating or idling. If the lights are on, they will dim briefly. If the fan is running, it will "skip" briefly with the sound. I have absolutely no idea what it could be...

I plan on taking it to a mechanic, but I thought I would post here to see if anyone has any insight. I don't know if it is an electrical issue, a fuel issue or something else...

Thank you...