99 Forester, 230,000 kms, 4EAT. There is the damnedest whining sound coming from under the hood when driving. When accelerating or maintaining a constant speed it is present, increasing in volume and pitch when under throttle. Once I take my foot off the gas and the motor is in a vacuum state the whining stops. Any ideas?

It's either resonance in your intake duct work (try rotating the section between the two filter boxes), or you have the "ghost turbo" from your power steering pump. It's not uncommon.

 

GD