That's a good question, it hasn't lasted more than a minute or so.
It's intermittent, I just tried it, and it's not ticking now . When the key is in the ON position, I can hear (barely) the fuel pump start for a second or so, then shut off. It's not doing it (rapid clicking/ticking) every time I start it, and it's the first time I've heard this sound in any of my cars or motorcycles with pump in tank. I have in tank pumps fail and they didn't always do it all at once. This sounds like a relay chattering, but I don't know of any in back of the car.
When I first heard it, I thought it was one of the seat belt motors, but there aren't any of them back there. So far it hasn't lasted long enough to pick through the fuse box to see what it is, or even get out and look under the car to see if there's some exhaust system part knocking on the body. The car does not have electric door locks, so it's not a lock actuator.
I am pretty good at trouble shooting, I spent a few years debugging code, and have been working with stuff with engines since the 70s. I was hoping that someone had heard something like this and knew what it was.
I'll check rockauto to see if they have the pump access plate gasket or seal or whatever it is, and get one and a pump. There are no subaru parts here, and it there was one in a junk yard, they would want twice of what a new one is.