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You have valve train issues.

When I did my head gaskets almost 3 years ago, I left #2 exhaust valve lash nut loose somehow so on first start up it sounded great then after 10 minutes it sounded identical to yours. A real pronounced thumping from the intake.

Its probably a pure coincident that it happened right after you were under the hood.
 

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Pull the valve covers to inspect rocker arms & check valve lash
 
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