Loud Single Knock on Start

[JustinF1997]

I have a 06 ram 1500 with the 5.7 Hemi TRX4 package and 105,000 miles. Most mornings when I go to start it there is a loud single knock that sounds like it is coming from the passenger side engine bay. It usually won't do it after it warms up a bit, but I can't figure out what it could be... it's been doing it for a good 2 months now. I have also noticed that it will happen while I am stopped at a light or a stop sign for around 10-15 seconds. Oil pressure is sitting good, just changed the oil about 3 weeks ago and routinely change it every 5,000 miles. Anyone got suggestions on what this could be caused by?
Sorry for the long thread also, thought I would try and get all the details I could on here.

 

[LoneWolf3574]

I have a similar knock or pop only when I started my truck. It took me a week or so to figure it out but it turned out to be my HVAC blend door changing position with the A/C on. I'd suggest having somebody else start the truck and you on the floor board under the dash. It may not be your issue, but it is an easy starting point.

 

[JustinF1997]

You know I think you may be right, LoneWolf, a couple days back I noticed that this only happens when I first turn the air on. And it was about 5 or 6 months ago I removed a flap because the motor was stripped out and at the time did not feel like tearing the dash out just to get at it, and getting that done was ridiculously priced at a shop. The flap I removed switched the air from recirculating to exterior air I believe.