Subaru Repair: Whistle noise when under load, 2003 subaru forester, subaru forester

Question
I have a 2003 Subaru Forester XS with a 2.5L engine and 86,000 miles.  I just bought that car last week and am not familiar with Subarus at all.  The car is running great, but when I am climbing up steep hills it down shifts and I hear this whistling noise from the engine almost like it's the air intake trying to suck in a lot of air, but goes away immediatly once the engine isn't under a heavy load anymore.  I pulled over to check it out and it only does it when I punch the throttle, not when I gradually increase the RPM's.  I don't know if this is normal when the engine is under load or if it's something else.  Thank you very much in advanced, I appreciate it.

Answer
Sounds to me like whom ever checked or replaced the air filter last didn't re-assemble the air box very well.Pretty common.If you look at the portion of the airbox that houses the filter you will probably find that the bottom half is loose.If that is the case its a fairly straight forward procedure but can be a little challenging. Lots of Techs struggle through it.Hope that helps