Stefan,
I completely agree with the other posts on this topic. We shot a sound sync film last summer with the Bolex EBM and it did present many issues. For us the biggest issue was the camera we had was screwed up and ruined all the film. That being said, the noise issue with the Bolex is problematic. If you find yourself using a Bolex and want to shoot sound sync, there are a few things you can do that will increase your chances of getting acceptable results. First off, get a Barney for the camera. You can find them used on line, and there is a place in California USA that still makes them new for the Bolex EBM, for about 200 dollars US. Also you can help eliminate more noise by wrapping an old leather jacket around the camera while shooting.
After you have finished your shoot, take the audio, and if you have a Apple Computer, you can get a piece of software thats called SoundSoap, made by a company called Bias. You need to get a copy of the original version 1.0, and for some bizarre reason, it does a really good job of removing the sound of a Bolex EBM from the dialog. We were blown away by how well it worked.
If you end up shooting with a Bolex, and have a Mac, and are willing to purchase SoundSoap, contact me and I will tell you how to get a copy of the original version 1.0.
But in the long run, it is much simpler to get a different camera.
Hope that helps.
-Tim