Dan, nice test. So under strain it makes the noise in 4th gear. Sounds like a drivetrain noise coming from the transaxle. If the noise occurs in 4th try 5th or 3rd same senerio.
If the noise is still there have someone check the transaxle. See if the fluid is clean ie. no metal shavings, severe burnt smell.
I just got a follow-up email asking me if my question has been answered. Unfortunately it hasn't. I've taken the car to a Subaru dealership as well as an independent Subaru shop and neither one has any idea what the problem could be. I did find other Forester drivers report the same issue on http://www.subaruforester.org/ at: http://www.subaruforester.org/vbulletin/f88/whistle-noise-under-power-58640/.