Miffed...but maybe thinking I'm not?

Here's what I got. 2000 Pete 379, Cat C15 Eaton 8LL. Sometimes there is a knock, sometimes there isn't. And the knock is never present when the truck is sitting still with the hood open!!! I know my clutch is going out, but I'm not financially able to just pop in somewhere and git'r dun at the moment. Now back to the knock...
As I said, it's intermittent and varies in volume/intensity. One second I'm driving along like a new truck, the next I hear Sanford And Son music as I feel and hear this god awful knocking. I've done the engine code check via the cruise control switch and only get "71-ignition switch power failure". That code goes hand in hand with my starter issue... The motor isn't smoking, isn't overheating, no water in oil or oil in water, no loss of power, not missing...nothing but a knock!
There is also a " ticking" noise that is constant and runs with the rpms. I've checked, double checked then triple checked the exhaust from the manifold to the tips of the stacks...no leaks visible by exhaust marks nor by running my hands around the connections/joints. I've put a bar from the engine to my ear like a stethoscope from top to bottom, front to back and since the knock isn't present when I sit, I can't find it. I've tried while going down the road pushing the clutch in and the noise is still there, putting it in neutral and revving and its still there and changing with RPM...then 30-50 miles later its gone for a while.
So, I'm leaning towards clutch is the cause. But was wondering what I should check in the meantime while trying to financially recover from an illegal tow that broke axle hangers and put me down for a month and a $6,000 repair bill I had to pay while the lawsuit gets settled. Any idears?

 

No smells of unburnt fuel. I'm fairly mechanical, but I've never even looked inside a diesel engine. I've worked on cars since I was in elementary school and was a crane service tech for 11 years. Maybe I can figure out what it's supposed to look like under the valve cover. If'n I was to open the valve cover and start the motor to see what's up, am I gonna take an oil bath like from a Chevy 350?

 

Another silly question...if it is in the valve train, would it not kick out an error code?

 

So, coming to my receiver today weighing in gross of 78,980, the knock was present the entire last 78 miles. After getting empty, the knock is gone! Came back when going up a hill, but for the most part, GONE! Guess I'm going to get a new clutch on Wednesday when I get (hopefully) back home. I Youtubed "valve lash adjust Cat C15" and will just do eet when I get home as well.

 

Oh, and the tick was an exhaust leak at the "Y" split. I put some goop over it that will most assuredly melt off, but its what I had in the truck to troubleshoot with. Tick is also gone.