A few weeks ago I noticed a knocking coming from the engine compartment during an COLD morning - below freezing. Initially the knock only showed when the truck reached 55ish MPH and would go away at approx 65 MPH. Over the last few weeks the knocking occurs at lower speeds and stays around much longer but only when the outside temp is below freezing. I have found nothing loose in the engine compartment, under the truck, or...... The knock only occurs when going down the road and does not have any set rhythm - I can put the truck in Neutral while coasting and the knocking goes away. Any Clues????