New 2015 Impreza with less than 1K miles makes a slight 1-2 second "rattling noise" in the engine compartment near the front drivers side tire. It only happens on cold starts when the car has not been run for several hours. Its coming from the engine area, but does not persist after 1-2 seconds. Any thoughts?

It's probably an exhaust heat-shield rattling.

These rattles can be hard to diagnose, and hard to eliminate.

My neighbors 14 forester does that. I hear it every night at about 11:30 when he leaves for work. Has done it since the day he bought it brand new.

 

Sounds like valve-train rattle to me. Similar to what happens when the anti-drain back valve in an oil filter goes bad and let's all the oil drain out of the supply ports for the heads.