Getz 1.4 squints and clacks

Hello dear forum, the problem with my Hyundai Getz 1.4, Bj 03/2009, I would like to describe to you once, because I was already in the workshop with it and allegedly everything is in order with the car. I bought the car six weeks ago with mileage about 86,000, before it stood about six weeks immobile at the dealer on the yard. Moreover, it is possible that the previous owner only drove short distances. At the first motorway ride I noticed then, that the s in the high speed range (i.e. at the entrance to the motorway) some grey smoke came out of the exhaust, which repeated itself during the next rides. A single time, after a long motorway ride in a rather low speed range with sunshine and running air conditioning, came on the exit while switching down a white steam cloud from the exhaust. I then was with the car in the workshop, on the occasion a complete inspection was done and… nothing found. The car mechanic could only assume that the gray smoke consists of residues (“dreak”, which would have dropped off in the exhaust, with the white smoke must have collected somewhere liquid. Probably this would soon lie down with the gray smoke, and it really only happens if I did not move the car myself or hardly ever a week. The white smoke, on the other hand, continues to occur after such rides as described above, also yesterday. The higher the speed, the faster the clacking. I’m afraid that this means a new visit to the workshop for me – but I can’t do without the car, because I definitely need it in the coming weeks. Do you have any ideas about what it can be? Then maybe I can find the workshop a mistake before nothing is found again… Thank you for your help Aljoscha