When I accelerate (most commonly up hills) there is a rattling sound that comes from the engine bay. It sounds almost like running along a metal gate (the diamond chain link kind) with a pencil sticking out so that it taps on every link as you run by. Crazy imagination, I know. Anyways, I came to the conclusion that it was air bubbles making noise someplace.

I brought my car in twice to fix the noise, and on the second visit, I had a tech take a ride with me. I was able to make the sound come out, and he immediately told me that the engine was taking in too much oxygen, and that it was a normal occurrence in Subaru vehicles. Apparently, our engines run very lean and can sometimes become more lean than normal, causing the noise. He told me that my engine needed to be 'pinged', which was a 10-15 minute procedure, but they couldn't actually get it done that day, since they were about to close up. Unfortunately, I have not brought it back since, but the problem has persisted.

After pulling the battery contacts off, and performing an idle relearn (http://www.uklegacy.com/forums/index.php/topic/128159-how-to-idle-re-learn-after-battery-disconnect/) the sound has lessened in volume, but is still there.

Also, do not know if it is related to the 'air bubble noise' or if it is coming from the K&N drop in air filter, but at roughly 1800 rpm the car makes a very deep growling sound. To be quite honest, it is a very appealing sound to me, but unfortunately I know that it is not a normal sound, and this worries me.

Update from May 10, 2016: 'Air bubble' noise can be heard in this video posted by another Legacy owner. I do not own this video, nor did I post it.

https://www.youtube.com/watch?v=4kZxZjhS4c0