Do you have access to a smoke tester? Something like this
I don't have one but may be able to get access to one. After a quick search I assume you are suggesting to pressurise the exhaust with it and look for leaks? Sounds a good idea anyway.
Sounds more metallic to me, not an exhaust blow. One thing that can sound almost exactly like that is a slightly bent tooth on the CPS reluctor ring just touching the CPS tip. Take the cover off the lower part of the bellhousing so you can see the ring. Check each tooth very carefully but I suspect you will see one that isn't quite parallel with the others and, most likely, with a shiny spot on it where it has been catching. I only know because I bent one of the teeth on mine slightly when undoing the flex plate to torque converter bolts and it made that very same noise when I first started it after the gearbox change.
I took the engine out around 3 years ago now so I would assume it would have clearanced itself by now? I did replace the CPS around 2 years ago but I don't recall any difference in the noise. I'll give it a check anyway. I agree that it sounds metallic though.
if that's the case then put the hose onto the cover and you will hear it. some times they are a bit hard to pin point , you need to push the hose right onto the manifold , yes it may melt a little but you will find it.
I'll have another go when I have more time. Maybe I went wrong by not having it close enough.