Lately I have noticed, I guess what you would call a "growling" coming from my engine. It is pretty loud in the cabin of the truck. It also seems to be louder when it is idling in gear, and a little quieter when it is in neutral. It has the ticking that I have also read about, I think that is just the fuel injectors, but like I say, this is more of a growl. Any ideas? It does seem to be louder when it is colder outside.