Subarus can make all kinds of funny noises and be fine. Mine has had some "piston slap" or possibly valve lifter noise since new when cold and it was quite disturbing at first. If it goes away after it is fully warmed up (15 min.) then ignore it for the rest of the cars long life. This is assuming you have checked your oil. No sense worrying about it either way - if it is going to blow there is nothing you can do to stop it.
There is a very good chance that this example is normal even for an older Subaru with a lot of life left (our 98' sounds almost this bad, but is due for a valve adjustment):