Thank you, guys!
We have decided to return the car and the dealer accepted, so we already got full refund. My reasoning was to try to tackle the proble, myself or take it to mechanic, the dealer could refuse to honour the warranty, if it turned out be a more serious problem or there was bigger damage. This is not a risk I am willing to take. If the dealers mechanic cannot find the problem, and in my opinion obviously there is a problem, then we will be looking for another car. Too bad really, because we both loved it.
Pavel