Thanks for all of the thoughts.
According to a carfax I have on it, the tb, water pump and tensioner were replaced about 25k and 5 years ago.
I started it today without the tb cover on to inspect further.
I didn't notice anything in there that would cause this noise.
I was able to verify again the knock comes from behind the exhaust cam gear. It's really bizarre, I start the car and there is no noise, about 1-2 minutes later the knock/ cluck starts and last for about another 2 minutes then goes away. I also noticed it fades in, gets progressively louder then fades away.
I don't believe it is piston slap but that is only based on a piston slap noise I had on an 96 subaru which was not the same as this sound.
In the next week or so, I will give it a change to synthetic oil and see what happens.
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