Did you actually replace the manifold gasket, or just tighten it?
A small exhaust leak often sounds like a valve tapping.
Dark garage, lights out. Use a strong pencil beam flashlight to look around the manifold area while you spray a little FI cleaner (make sure it says "safe for O2 sensors") down the Throttle body - while it's running of course.
The cleaner will make smoke in the exhaust system. The exhaust system should be tight and carry all the smoke out the back. If you see smoke in the engine compartment from it, you can probably see where the leak is.
Listening through a hunk of heater hose like a stethoscope helps locate the noise too.
Or remove the rocker cover to use the hose, and/or put your fingers on each rocker, one at a time. Your fingers will feel the "bump" better than your ear can.
Betcha it's still an exhaust leak.