Engine vibration

There is a vibration and growling noise in the engine of my 2002 Trailblazer. The coolant level was low, but I could not find any leaks. The check engine or service engine light has not came on at this time. I just had the fan clutch replaced. Has anyone had this problem and if so, what was it?

Kelly - did you ever get this fixed? This exact thing is happening to me now.
Anyone else have this problem and were they able to diagnose it?

The other possibility is the idler pulley below the alternator. I had a friend that had the idler pulley bearing started to fail and made a similar sound. Replaced the idler, noise gone.

Remove the serpentine belt see if the noise goes away, if so suspect ac bearing ,belt tensioner bearing, alternator. anything the belt drives.