Just wanted to post an update to this.
Spent today replacing all 12 mini V-wheels on my Predator. I can safely say I found the cause of the noise...
Eight out of the twelve wheels I removed had bearings which were severely grating. I determined this by removing the belts from the steppers and being surprised at the noise of the delta tower carriages moving on their own. All of the wheels have been replaced with genuine OpenBuilds Mini-V delrin wheels. I opted for those over the poly-carbonate as they're generally considered better for non-heavy duty applications.
After getting everything back together, my printer is now much quieter. I've also lowered the currents on all 3 axes down to 1600mA. This still seems more than enough, but more time and tweaking will tell.
Before the change, moving the Z faster than 200mm/s would make unbearable noise. I've now tested it up to 400mm/s (with significantly lowered current to prevent back-EMF) and all is A-OK.
Thanks to everyone for your thoughts and help. Hopefully this post will help others in the future