The iMac 27" has been known for these problems.
Although they are becoming less and less common these days. The Mid 2010 iMacs barely have these problems.
I purchased a 27" iMac i7 mid 2010 model brand spanking new from the online Apple Store and it's running deadsilent with the most astonishing screen I've ever seen.
So, in some rare occasions these problems do show I guess, but not in largle number anymore. Of course, when someone has this problem, he'll post it on the forum(s). Thus, it may look that they are still there in large numbers, but that's just because most happy customers are happy in silence, I shall say.
Every product has it's flaws and every product is being critizied. Apple products are known for attracting much more attention than other brands, so flaws are
always overhyped.
Overall I got a perfectly good working machine, and I've always had this with Apple products in particular. Just do it, and experience it for yourself.
So, it's safe to buy and you will
absolutely enjoy it.