I own a 2000' Accord 4-Cylinder 5-spd with only 35K on the odometer.
About a month ago, I noticed a metalic rapping noise coming from the
engine bay. At first the noise would go away after the engine warmed
up, but progressed to intermittent noise when the engine was under
light load usually under 3000 RPM. It could be duplicated with the car
in neutral and reving to 3000 RPM  tried several tanks of high test,
with no change. I checked everything attached to the engine such as
heat shields and found nothing loose. I finally had to bite the bullet
and take the car to the Honda dealer. Several days later , I got a
call saying that the noise was coming from the lower end and that the
pan would need to be dropped ($170) The next day, they said the head
would need to be removed. Several days later I was told that Honda
recommended replacing the engine block. They would pickup yhe cost of
parts and I would have to pay labor ($1700). They never did pinpoint
the source of the noise. So now I have a brand new engine from the oil
pan up to the head. The car runs smooth with no unusual noises. The
total cost of parts that the factory picked up was $3700. This
included a new throttle body. The old one was damaged during the block
swap. I still can't understand why a Honda engine with such low miles
and proper maintenance could develop such a problem. I'm wondering if
the fact that the car would sit for months at a time without being
driven had anything to do with it ? I bought it in 2002 with 27k on it
and in 4 years only put 8k on it. I intend to drive it at least once a
week from now on.