The Vax Zen is the quietest one I have. The Dyson Dc07 is by far the loudest - mostly the howl from the cyclone assembly itself.

Any cyclonic vacs with a power dial selector don't work properly on low speed - although they are quiet on low speeds, the cyclone doesn't work well at all, and the dust just clogs the filter quicker.

Only bagged vacs should be fitted with variable motor speed, so they are always much quieter in use than cyclonic and more effective at low speeds.

Don't forget, the Zen has one speed - maximum at all times, so its effective at filtering the dirt. The Bosch has variable power to make it quieter, but has an old fashioned cone filter system which clogs quickly, so the Zen is a better machine, even though it is made in China.

I only used it once - its now back in its box in the loft. I'll be keeping it in long term storage, but not using it. I prefer to use a bagged machine for daily use.