Clattering noise when put into drive after HCI?

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[TD="class: alt1, bgcolor: #F8F8F8"]I recently did a HCI swap and drove it around for a little bit to make sure everything is okay (which it isn;t). Anyway I drove it around and after about 2 miles it was making a knocking/clattering noise. So we took off the valve cover and found out that my roller rockers lockers weren't tightened so my rockers loosened up and were bouncing around. I went through all of them again and adjusted them all again and made sure I tightened down the locking nut. SO after that I started her up again and she idled 10 times better in park. I got in and put her in D and the same clattering noise was back. So I put it back in park and it sounded fine. I revved it up and it sounded great, no clattering noise. After i let it idle down to normal, I put it in reverse for s S%*its and giggles...well to my amazement there was no clattering noise. After in reverse I put it in drive and the clattering noise was back. So I'm just trying to figure out where this clattering noise is coming from when I put it in Drive and I'm at idle?
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Before when the rockers were loose I would rev it and the clatter would get louder. But when I rev it now the clatter goes away (only in drive the clatter happens)

 

Is it possible for my flexplate to become loose eventhough I never touched it?