I can't seem to identify this nose!

[daboxsta]

I'm usually pretty good at figuring out noises. However, this has me stumped. It kind of sounds like crickets! At first I thought Accessory drive belt. Changed it. No difference. Then thought about idler pulleys. Had my shop check, and was told they were fine. In fact they said they couldn't even hear a noise.
I'm not crazy. When I pull into an enclosed place, or pull up next to another car, it's plain as day. Not loud, but definitely there. Off idle, it goes away. It's not rhythmic. sort of random. The pitch even changes. It's driving me nuts! Anyone have any ideas? I'm getting tired of paying diagnostic charges, and being told they couldn't find anything!