When I got home from work today I walked around to the back of the Xterra and I heard this buzzing (or more like a sign wave tone) on the passenger side rear. I looked underneth and all but can't tell where its coming from. The X has of late been throwing a P0420 code. She runs perfect so I just clear the code till I can deal with it. Any ideas what might be causing this noise I'm hearing? I'm thinking it could be a air leak making that sound. IDK.