C4 HDI 110 -strange noise when de/accelerating at lower RPM

actually i'm facing a much more interresting quest than being sorry about a built in RD4, what's unable/not supposed to play MP3s...
Actually, having driven my C4 HDI 110 about (only!) 350km now I often notice a whining noise or sound, combined with a sudden, but slight decelleration hit - about 1-2km/h less on the speed meter. Hard to describe, but it sounds almost like "stopping a slowly free turning bicycle tyre with a bare hand"- a rather silent, bit grinding and high pitched sound going from louder to silent... The local Cit service is clueless. However I'm not alone with this phaenomenon: http://www.motor-talk.de/t606852/f187/s/thread.html . In my case, the noise begins (=total duration about 1 to 1,5 sec., combined with a sudden but very low deceleration shock) when accelerating or decelerating from lower RPM (=low, but far away from stalling the engine: about 1800 to 2300 RPM at the 3rd or 4th gear). It even may start suddenly when driving for a while on a straight even road with low throttle (=diesel consumption meter shows about 4ltrs per 100km) at 70 to 90 km/h at 4th gear. The effect does not appear on neutral gear while standing or when the clutch has been pressed down. The service here promised to get more informations on this... but they guess it's some harmless sort of resonance- effect, caused by the vibration or fuel pressure at lower RPM. Would be interresting to hear if other C4 HDI drivers (or even Benzin/Gas drivers) noticed this noise before...
Best Greetings,
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