A couple of times when i have started off from a stop the engines lets out this low growl/roar sounds as I take off.... its only happened twice - once when it was cold and once when it was warmed up..

It really sounds like a mid-range or larger truck when the cooling fan comes on.. but i dont think that is it..

Any ideas??

 

I'll bet that's what you heard. When its warm out the clutch fan will come on just about every time you come to a stop sign or just slow the RPM's down to idle. It will roar loud as you take off for a second then quiet down depending on how hot out it is.