It's very loud down in the wheel well so I think you're right about exhaust manifold / cats, especially if you just worked on them.

When I had the transmission off I had the rear O2 sensor wiring all down and out of the way. When I put everything back together, there was a horrible tapping noise, not unlike what you have, though I don't really remember. It was absurdly loud though. I was panicking but then I noticed it changed if I slapped the valve cover on top of cylinder 6. I kept looking around and eventually found the wire harness bracket back there was about the source of the noise. A couple minutes to get everything repositioned right so it stopped and all was well. So I would suggest checking all those brackets, the rear O2 wire sensor brackets, which you had off with the head right, the exhaust brackets etc. If you leave it running before it gets hot and just start hitting everything you might be able to find what it is.

Good luck. I have the head off of the m52 on my E36 now and it is just a monumental amount of work. Respect. It is taking me months to clean things. Tonight I was supposed to remove the exhaust manifold from the lower end and get them out and pressure wash the cats out but that didn't happen.. maybe tomorrow

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'05 E53 3.0 6mt
'17 F30 340ix 6mt
'96 E36 328is, in progress