Vibration noise from ... the roof?!?

[AHels]

So I'm on my way home this afternoon and I get stuck behind what can only be described as a convoy of road-trains..

Anyways, we have a stretch of road where visibility is easily a mile, so I hit the accelerator and start overtaking..

Everything's fine until I hit about 95mph and I hear this ... vibration noise from the roof.

Seems to come from right above the steering wheel, so I pull over at the next restarea to check it out. Figuring it'd be the antenna vibrating in the wind, hitting the roof. I have a look and the aerial is pointing right up like always.

Check the marker lights (which seem like they're a bit loose, but I seem to remember they've always looked/felt that way). Check the cross bars are fastened properly (moved them last week as I transported some wood), seem fine. What I do notice is the actual cross bar mounts (ie. the tracks) seems loose at the front. I don't really think this would cause the noise I heard, but well...

Checked the TSBs but all I found was

Reports are being received on customer complaints of roof rack rattles. The cause of this condition is that the cross bar mounting screws on the passenger side are loose. They are intentionally left loose upon shipping to allow for any necessary adjustment upon installation of the roof rack assembly at the dealership. Be sure to tighten these screws to 4.5 Nm or 40 in. lbs. upon installation. Refer to step #3 and figure #6 of the installation instructions.

which doesn't seem to be the cause either.

So, any ideas on this? Am I missing something obvious? It wasn't even that windy - and the noise reproduced easily when hitting those speeds again.

Should be noted that I regularly push the limit of what the H2 is capable of on that stretch of road and never heard it before.