Hi there!
A Heritage II/Legend/Legend II would be somewhat quieter. You would want one with an amodel fan and with you bearings in good condition for minimal noise. Try it before buying. If you want to stick to Kirby G-Series, I suppose you could get a Diamond Edition—the only Kirby with a manually switchable motor speed—and use it on the low speed setting. That said, I would be careful doing so; “low” speed is not intended for steady use and might cause problems.
If all you are after is a tough, quiet, high performing vacuum, check out clean air commercial vacuums such as those from Sebo, Windsor, Sanitaire, Hoover, etc. Many of the commercial brands offer units advertised for their low noise.
