Pull the covers and see if there is anything noticible in there. Could be a loose bolt knocking around, or a rod punching out the side of the block. Don't know until you look.
Even if you take it to the shop, they will run it to find the sound. Let her sit and idle for a few and try to track it down. Once you have the location, you can go from there.
Thuds and knocks are not good, but you will know what it is.
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