Strange Whistle/Groan noise in 2006 Toyota Mini Van

Just recently I’ve been hearing the strangest noise coming from what sounds like in the inside of my van near the middle row of seats. It is this weird whistle/groan sound. It sounds very similar to those “groan tubes” that you turn upside down and they make a weird noise. The whistle/groan doesn’t last very long (maybe 2 seconds) and the problem is very intermittent. The sound occurs when I am just driving along (neither accelerating nor decelerating). Has anyone had any similar sounds? Any ideas as to the cause? And, yes, I’ve cleaned out the van and checked that the kids did not leave any noise maker toy in the van!

I suspect putting the car on a lift and taking a look would turn up something.