No expert on creaking noises here....BUT, I wonder if it's possible that the roof attic itself could be vented {as it should be} and is building up pressure because there's no "flow-through" due to the lack of soffit ventilation?

One who enjoys helping with roof problems, for the fun of it.

Unfortunately the soffits were vented previously and the creaking noise was still there. We thought that reducing the ventilation might help, but it doesn't appear to.

Another possibility allowing unusual draft into your attic could be open soffit{s}.

Don't know the configuration of you roof, but if you have any soffits which abut adjacent roof areas, such as a dormer or other similar structural feature, the soffit{s} may not run flush to the abutting roofed area. Have seen a number of situations where the soffit was installed from a few inches to 1 ft. short of being flush to shingles, leaving a gaping hole, allowing critters and birds access to the attic.

Many times these "holes/openings" cannot be seen from the ground.

If no way can be found to eliminate the wind, keeping iut from rattling the insulation, and it's unbearable enough, drastic measure might be to access insulation from interior drywall in order to reaffix it. But...that's a pretty big deal obviously.

A roof CAN be your friend