hiya mate, ive also got my drobe in behind a normall looking sliding door.
them fans aint the quietest things in the world

. with a bit of reaserch on here i managed to get my noise down to a level that i feel is acceptable. fair bit of fucking around with it and i still find myself wondering if the neighbours can hear it

.
good luck with the 250!! my drobe gets a little bit on the hot side using only a 125w enviro but im sure you can overcome this with beefed up extraction and a cooltube or something.