I'm very well aware that the helicoils are steel... when I said aluminum debris I said that because that's what you'd most likely have dropped in; helicoils are very strong steel, it's not easy to break them.
I mentioned a valve sticking due to debris preventing it from closing... it's not unlikely at all if a curl of aluminum entered the cylinder when you were tapping it. Aluminum is light and the air would readily carry it with. I'm not saying that's what it is... it is possible.
You could pull the valve covers and check the shim clearances on the exhaust side to see if one isn't fully retracting... if it's caught on debris I'd expect a 1mm or more gap. It's an easy enough process and can be done quick enough if you're only looking at one or two cylinders.
A rough idle after just swapping plugs is usually due to either a problem with the plugs or the wiring you had to move to get to them. I don't think it should ever sound like a knock though; Not unless the bad plug was keeping the cylinder from burning to the point that it was just hiding an existing rod knock (but that's a lot lower than the head as you know).
Cheap Champion plugs now.... sigh.
I gave you my 2,000 cents worth of advice, do with it as you will.
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