For later EJ253 SOHC owners, that engine doesnt have ANY lifters at all - hydraulic or solid. They have roller cam followers and a version of VTEC. Any loud VT noise when warm (independant of load) would be excessive valve lash or injectors. The rockers have a jam nut adjuster, IIRC. Be carefull as too tight is MUCH worse than a bit loose.
Correct on the lack of lifters... the cam(s) operate directly on the rocker arms. They do not have lifters as typically experienced in American cam-in-block engines. I would not mess at all with valve lash unless you are previously experienced, and regardless, it's likely not the source of your noise anyways.
As mentioned, piston slap is most likely the cause, and is of no real concern anyways, partly because unless you're going to tear the engine down and put new pistons in after measuring tolerances, it will never get quieter.
But that's a dubious point anyways, as many Subarus have gone hundreds of thousands of miles without incident... people get to tolerating the noise when they see it's not negatively affecting reliability. FWIW, the Chevy LS engines are notorious for piston slap as well, and they are considered bulletproof. I have an '09 Pontiac G8 GT, and even at 9k miles, when it is less than 30*F and the engine is cold, the piston slap is so loud you'd swear there's an elf in there with a small ballpeen hammer! Never used any oil, never had any problems.
Enjoy your car... it will give you a louder noise than a little tick or ting when it's unhappy with something :) LOL