2004 6.0 with 328k miles, just bought it about 3 weeks ago, no issues other than some minor oil leak. Changed oil and filter. Added some fuel cleaner two tanks ago. No lights on the dash... two days ago I noticed an audible knocking sound while idling, then more when I’d come to a stop with no load. If I’d be under load it’ll go away. (Or even just put it in reverse from neutral) Driving is fine and quiet at any speed, no miss, no power loss, but the knock/rattle sound is there as soon as it’s back at idle. There’s no smoke other than a small puff at a cold start up. It’s just so loud I can’t ignore it. It’s definately louder on the driver side. Any ideas what this could be?