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.