So after my EJ22 swap into '97 2.5GT I notice a medium-pitched tone coming from under the hood, almost like an electronic test tone. It doesn't vary in pitch with RPM and it's not a whirring or scraping, more like a ringing sound. I only hear it with the engine running, not if the ignition's on but the engine is stopped. I poked around with an old stethescope and found it loudest in the air filter box, then I traced it up as far as the MAF sensor- it's either coming from the MAF sensor or somewhere else in the intake path. Anyone experienced anything like this? I'm relieved it's not a timing belt idler or worse, but I'm really puzzled!

double check to make sure the air filter box is seated properly. it can be a pain and leak, check the bottom edge closet to the engine.

check around the ABS unit/HCU which is right next to air box...Ive had some strange noises resulting in problems with that...

My 97 Impreza did that. I double and triple checked and it had no leaks, it just always did it. I even had a subaru tech take a look, and he couldn't find anything either

Found the problem- it's a gray cylinder with a 3-wire plug that sits on the intake manifold right under the TPS and has about a 3/4" hose that connects to the plastic intake plumbing. Not sure what it's called, but I noticed that the one on my '96 Outback makes the same sound just not as loud.