well for me it happens even when car is hot, and as i said since day 1. i also noticed the same when i rented an impreza in Hawaii which was a sort of hot place. from my experiments while monitoring oil solenoid duty cycle, the howling occurred at the same time i got the lights. so this might be related to high lift Cam switching lobes (anywhere between 1500-3000rpm at over 80% engine load).
this is considering that we are both hearing the same sound.