My cousins (the don't live in Switzerland) have been in the parquet business for over 20 years now and have around 30 employees. They only work on whole new buildings. I had the same problem with walnut parquet at my home (not in Switzerland, again) but mine was squeaking on certain places that were affected by the sun. The recommended that those areas should be taken away and put back with better glue. What would be even better, you sand everywhere (the top of the parquet) and you fix those areas where it is squeaky, put back and that is it. Ask what kind of price that would be and if it will solve your problem, since not every wood/parquet is the same.