I get the same problem in a 6 month old R22 Raven II, mostly with just me in it and full fuel.
A little bit of pedal seems to make it go away. It's definitely an aerodynamic issue, nothing to do with torque. I suspect it's also related to when there's a reasonable quartering tailwind component.
The one I fly has plain windows (not bubble) - I was told that the wing-effect pulling the doors open and the reduction in VNE made it not worthwhile.
I'd love to know what causes the wag - it was a cause for some concern the first time it happened! Several people who regularly fly older 44s didn't seem to have ever felt it, so maybe it's a feature of the later models?
Lafite