2003 Blazer "growling Noise" with start up

I have 4WD chevy Blazer 6cyl. 2003.Little more then 50,000 miles. when I start the vehicle I hear what I can only describe as a GROWLING noise. It seems to happen as the RPM's come down from intial start up. I also can hear it as start moving the vehicle but as I speed up it seems to disappear. I just finished getting my wifes isuzu fixed up, with a whole lot of help from AF members, and now I'm hoping you guys can help me with mine. I don't hear everytime I start the vehicle it had to rate I would say I hear about Half the time. I have had no noticnle issues with vehicle since I have had it for about a year now except some squeking in the front end, I found the picture pointing out all the grease fittings in the front end on AF and hit most of them and that seemed to resolve that issue, but this one I can't figure out.