Not much to go on, but I have kind of a sneaking suspicion that your HDD is in the very early stages of failure, and there really just aren't any good consumer level diagnostic programs for this.

So take the unit to an AASP or Apple store, and INSIST that they run the like 2 hour long full surface scan test for the HDD. They aren't going to want to do that, because it takes like 1-2 hours, just be persistent. Don't let them fob you off with their AST "diagnostic" which is just this side of useless for anything more than finding bad batteries or AC adapters. So if the test only takes about 2-3 minutes, tell them to check it in and run the full battery of ASD tests on it, including the full surface scan of the HDD. Tell them something almost exactly like that, and make sure you use "ASD". If that passes, then at least you know the odds of your HDD going south are very remote. If they don't know what ASD is, then thank them and take your computer somewhere else. You don't want them within 10ft of your computer if they don't even know what ASD is.

Prepare accordingly, since it might be that there are a number of units ahead of yours for someone to look at.