I'd bet it's not a valve but the timing chain. See if the sound is coming from the front of the engine on startup.

Are there new plastic chain guides? A new chain tensioner?

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FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered