Explorer Belt Noise

I have a 1998 Ford Explorer. When the engine is idling, the engine area eventually starts to produce a load droning sound, perhaps from a belt. It has seemed to have started up since the weather started getting warmer (central florida). I have had this in the past. I mentioned it to the dealer a year or so back and they said the serpentine belt needed replacing, which they did. I took a trip immediately thereafter, and it continued to make the noise for a while, but it may have subsided as the weather changed to cooler months. Does anyone have an idea what this is? Do I need to replace the belt tensioner? Thanks, Roger