Altaplex door flap flutter/hum

Ear plugs? (Slightly facetious, personally I don’t like to use them when camping but maybe you can get on with them.)

If you change the mass of the part of the door that oscilates can possibly get it out of the resonances. If you use poles you can take the one not used for the shelter and put it through/attach it to the loop I think you have halfway down on the door and let it weigh down the door with one end resting on the ground. This can possibly be more effective than trying to tighten every thing even more. But the pole must be attached to the door tight enough so the door is not allowed to move in the oscillations without bringing the pole with it. This does´t always work, it might just change the frequency,  but it may also stop a resonance. If you dont have an extra pole you can use someting else with some mass to hang there. Basically to do a job the mass of the zipper may do for other tents to avoid simular resonances.

I actually already always wear ear plugs. I am just trying to differentiate a gust of wind setting the tent a-buzzing from a Kodiak bear voicing its displeasure with me when I am trying to sleep. :^)

The idea of hanging something weighted from the door (like a small rock in a stuff sack) sounds like a solid plan. I already use both trekking poles as shelter support.

Is the fluttering door the upwind door?  Seems a little counterintuitive but maybe loosening the door would have an effect?

Is your tent a more recent model with the toggle on the doors? If not, you can add one for a couple dollars.

The tent was pitched so the door was basically downwind, though the wind shifted direction somewhat during the evening. I left the vestibule flap that was more upwind deployed/tensioned and left the downwind side open for views and ventilation. Standard pitch stuff.

I do have the mid-door toggles, and already sewed on another small loop right on the fabric edge on the right-hand flap (where the Zpacks-provided toggle is inset a few inches) for flutter control. It’s possible that this will just create two shorter lengths to vibrate (like placing a finger over a musical instrument’s string on the fretboard), but we’ll see. It was fine in a steady 15 mph wind, but was set to humming in 20+ mph gusts.