Is your xf a manual equipped car? Mine rattles a bit when I shut it off, this is due to the springs in the clutch. Nothing to worry about.
As far as a tick up high, probably a lifter problem. A tick at first start up, but goes away within a few seconds is not a big deal on an engine with some milage on it. If it continues to tick then you probably got a spec of crud in one of the lifters that is blocking off the supply hole for the oil. It may or may not work itself out.