The first one could be due to the fact that your system's fans are caked with dust, and so aren't being as efficient at moving air through the case for cleaning, however, it is a P4 so it's entirely possible the fans are just normally loud. Better to have some white noise than putting your computer somewhere that muffles the noise and also prevents proper air flow, which makes it overheat and fail.
The second issue could be a number of things. XP's power saving functions were never all that reliable, so you might just be better off shutting the thing down instead of using any kind of sleep mode. It can also be the sign of bad caps on the motherboard, which is quite probably why the thing was a refurb in the first place. Electronics tend to fail more and more as they get older, and you bought some circa 2000 equipment. Refurb or not, that's still about 10 years old, so the odds of something going wrong start going up exponentially. Plastics become brittle, thermal grease dries out, capacitor gel starts becoming chemically unstable... All the little manufacturing defects in every component, which weren't a big deal when they were new, start catching up to you the older the system gets. It's just a fact of life you will have to get used to. If your second issue is one of bad caps, then eventually the computer will take multiple attempts to start, and then some time after that, it will simply fail to start ever again. This could be tomorrow, it could be 10 years from now, but it will almost certainly happen, so just prepare yourself.
i don't know much about computers but i can learn!
it is a windows xp, refurbished. intel pentium 4.
as well as the sound, sometimes when i put it in stand-by
the screen doesn't come back on. any ideas?