I've noticed on my '01 sport trac that sometimes when the truck has sat up that it makes a growling noise when I first crank it. It sounds like a diesel truck starting. Any idea what this might be?

 

Depending on which side the noise is heard on (usually Passenger) you probably had exhaust manifold bolts that are broken. Does it always start off with that noise and once warmed up it is minimal and when accelerating when cold it is louder than when first started? It is typical that the first few bolts (Front too Back) break off from corrosion. Hope this helps.