Mine slaps from cold all the time until the oil is up to full operating temprature, it's a bad piston skirt design that's to blame (was rectified on other years) but due to the lay out of the engine most of us suffer a little slap when it gets colder out,
It sounds like a high pitched metallic clapping, if it's a dull thud or an irregular nose then it's probably knocking as opposed to slap,
A good map won't cure it, as it's only when the pistons have contracted slightly and don't 'fill' the bores like they do when warm.
As for the shock are you still looking at the forester ? Or am I confused?
90%of foz's have sls suspension which dies and leaves you with saggy !Removed! syndrome, looks a bit like a dog sitting on its rear when the car is stationary,
You can grab the rear arch and lift up, assuming it's not full of rust, to unload the suspension, if it drops loads when you let go the shock is dead,