Ph1019

New 2014 outback 2.5, engine makes humming.woo,wooo,wooo, jet engine sound at exactly 3700 rpms, goes away above or below 3700 rpms.will also make the sound in neutral. Dealer could not figure out. They started checking other cars on the lot and they all do this. They had me meet with Subaru engineer and his legacy 3.6 made same noise. They cannot tell me what this is only that it is a "normal characteristics " very disturbing. Anyone ever heard of this? anyone experience this with outback or legacy 2.5 or 3.6? Should I be concerned long term w/car?