This morning my i7 iMac 27", after awakening, started to make a strange, high pitch noise, not quite the usual very low one nor the typical fan noise, nor the "reading disk" one. I have many years of (just user) experience and can recognize these, and it was quite diferent (and threatening). I started Techtool's Deluxe 3.1.3 as given in Apple Care CD and instructed it to do all the tests. It "passed" all the tests until "Surface Scan" was reached. Then it took hours to go through it, and presently it is half its way (step 6 millionth of 15 million) and has already found more than 14000 errors. At this point, each step gives a new error. It keeps going, but I understand such errors mean the hard disk is failing and should be replaced.
Since the computer is still under guarantee, I wonder whether I should ask Apple for a replacement right now ?
Present: MacMini, MacBook, i7 iMac 27", Mac OS X (10.6.4), Mac-fanatic since 1987