2002 Eurovan intermittent whistling noise

I started and drove the van yesterday and was greeted by a new noise. High pitched, sorta like a penny whistle or a bosun's pipe. At first I thought maybe the brakes, and then the throttle, but after a while it seemed there was no connection to either of those. The longest warble lasts maybe 3 seconds, and there's also shorter chirping, all with ascending and descending pitch changes. The noise is intermittent, pops out perhaps every ten minutes or so. No connection that I can tell with engine RPM or vehicle speed.

Any ideas?

Thank you!