Ours had the same issue on our front vent, over the cab. The whistling (while driving) nearly drove me mad!
We fitted a 'vent spoiler' which solved the problem.
During winter we have spongy square pads that fit in the roof openings to stop wind and keep the heat from escaping through the permanently vented roof vents. It's a semi-permanent solution, in that, we have to remove the fly screens to insert the pads, then reinstall the screens. Downside to this is extra condensation and a bit less light, so we've rarely used them.