I have a 2003 Outback/4cyl/auto with 42,000 miles. The piston slap noise started this winter and it's getting worse. Under warranty, the dealer replaced the timing belt tensioner and the nose remains. I now have an appointment with the zone rep coming up soon. And this car is a replacement for a 2002 that was returned as a lemon.
I also have a 1995 Mazda 626 with 226,000 miles and that engine doesn't make a bit of noise. So Subaru has a big problem here.
Back in the 60's, when I was in high school, only worn out engines had piston slap.