Hello, Taylor here.
I’ve got a 2003 Subaru forester with 65000 miles on it. I bought it recently and I never realized it when I bought, but when I get up to about 75-80 mph there is a whistling (not a screech I don’t think). It sounds like it’s coming from my vents but I can’t be sure. I just had my oil and final drive fluid flushed and changed, so it’s not a trani issue, or belts, they looked at those. It’s nothing in the air filter either. Does anyone have any idea what this might be, or how to fix it? Is it ram air?
It sounds like a simple wind leak. The air for your internal ductwork comes in through the slotted panel in front of the windshield, and any leak anywhere in the path, including the ductwork, could manifesst itself as a whistle.
Another possibility is the intake plenum, that path through which the wir goes from the intake behind the grill to the throttle body.
And if you have a sunroof that’s another common whistle source.
Wheel well aprons and/or mudflaps can come loose and cause whistling.
And, the neoprene gaskets around doors and windows can get damaged and cause whistling. Check the gaskets for abrasive wear.
And, finally, you could have something hung up under the car.
definitely not the sunroof or doors and windows those are so worn the wind goes right through 
but the ductwork leak and/or the plenum I will have to look into. is there an easy way to find leaks in the duct work or intake plenum without getting involved with a mechanic?
I believe that model is prone to a whistling noise which is actually from the side mirror at high speeds. I have heard of remedies from vaseline to duct tape.
No dissrespect meant, but if those are so worn that the wind goes right through why would you not consider them prime candidates for a wind whistle?
Well, the sound is definitely coming from the dash, not the windows. Firstly, it happens even with the windows down. Second, I had a friend do a ride along, and he confirmed that the sound appears to be coming from inside the dash possibly towards the passenger side. I also just found out that my Cabin Filter is missing, might that be the whistling issue? I cleaned out the intake in front of the windshield, and I may replace the Cabin filter and see what that does. Thanks for your help.