Loud noise under dash

[Mikerson]

Hi folks, I wonder if you can help me identify a problem on 2001 E-250 5.4L.

We took the van on out for one last adventure on Sunday before the snow starts to hit. While driving about 35 on a paved flat stretch, there was a loudish thump from the passenger dash/footwell area. It sounded like somebody dropped a full water bottle onto the rigid plastic of the footwell area. Temps were in the 20s and we had only been on the road for 15 minutes so the van was on “mix” defrost and floor at full fan and max heat. Immediately after the thump, there was a whine sounded like it was coming from the lower dash in front of the passenger seat where the glove box would be on a car. Sounded like a small motor spinning freely, or maybe when you turn off a shop vac that is on its last legs. I was like “What the #&$@ is that!?” and thought it might be related to the HVAV system. So I turned the selector knob from mix to off, and that stopped the sound. After that I tried to recreate the problem by turning back to mix and defrost, but the sound did not return. We continued for another 30 minutes to our destination, parked/hiked, and then returned home without having any more noises or observable symptoms.

Has anyone else experienced this and have a fix? I am generally aware of the vacuum reservoir failure/fix, but thought my year was not among those affected. Could this be some kind of ac compressor issue as well?
Thanks in advance.