Anyone have a humming sound coming from around the air filter box? This is a loud hum, though you can't hear it inside the car. I finally pinpointed that when you open the top of the air filter box it gets louder. So the source is up there somewhere. The sound does not change with engine speed. I tried revving and it didn't make a difference. It is a perfect pitch, sound like a tuning fork. Any ideas?

 

96 2.5 outback

It's normal. The noise is coming from the IAC valve. It's a rotary valve that oscillates back and forth to control airflow into the engine.

I tried tightening hoses and it certainly made it quieter. Much more bearable. I think the hose that went to the MAF was not on all the way. Seemed to make it worse.