I would check a couple things. My truck has TWO knock sources. First being the power steering pump. I heard this knock and immediately shut it off and started yanking on the pulleys. Took all of about 2 minutes to find that my power steering pulley has TONS of slop. I actually just ordered a new pump today because the pulley installer didn’t tighten it down any.
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The second knock is from my flexplate. I had friends of mine tell me “your bottom end is gone”, “your cam wiped a lobe”... well none of them were even close. I pulled the access shield on the bottom of the bell housing and I saw clear as day, the old girl has a crack on her flexplate.
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Here is the noise....
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I will be putting a new one in and a high stall converter next week, BUT this noise varies... I sometimes don’t here it for a long time. Sometimes it randomly starts clacking again.
If it were my truck, I would do some looking on the pulleys first.