99 honda civic rattle

I have a 1999 Honda Civic LX 4dr with 199,900 miles. Two things:
First, and most worrisome, occasionally when I accelerate there is a metalling rattling noise coming from under the hood. Imagine a single
coffee bean in a grinder - that's the closest thing I can think of to compare it to. It's inconsistent and only happens at higher RPMs (around 3000 +), and it is most pronounced when going up hills, like freeway onramps.
I have no idea what it is or what to check... and it worries me.
Second, and less worrisome, the engine makes a noise when accelerating with a cold engine. From reading online I gather that this is probably a lifter tick, or the piston is slapping, or something unimportant along those lines. But here's the kicker - when my engine is running I can feel a distinct knocking in my #4 cylinder when I put my finger on the spark plug cap. Only that cylinder, all the time - it's always there.
Suggestions? Much appreciated.
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snipped-for-privacy@gmail.com wrote:

make sure the spark plug is screwed in properly.
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