Headlights dimming while driving & while idling, accompanied by droning noise...

[khelms_98]

I drive a lot at night and recently, on my 2001 3.0 w/ 90k miles, I noticed that my headlights are dimming at regular intervals (every 1-2 sec) while driving. The dimming is hardly noticeable but not really that bothersome as they don't dim enough to cause driving problems, it's just a slight dim that caught my attention. However, when I stopped at home and got out with the engine idling and the headlights still on, I noticed that every time the headlights dimmed, I could hear this droning-type noise from the engine. It wasn't a horrible, loud drone, just minor, but it accompanied the dimming of the headlights. It almost sounded as though the engine was slowing (at idle), however, the tach doesn't budge one bit. Could this be an alternator issue or a weakening battery or something else? I bought this X5 used back in May and I haven't clue as to how old the battery is. There are no warning lights whatsoever and the engine runs fine, always. I'd greatly appreciate any input. Thanks.