clapping at partial load

Hello opera friends! for a few days I have noticed that my omega b in the partial load range (1400-1800) is only slightly clapping with warm motor. the noise sounds like the clapping when you speed up undertour. since the motor has hydrostössel, it would be logical that the hydros rather clapping in the cold condition, but that doesn’t do it at all. then have opened the hood and played in empty run with the speeds (cold and warm), but there is no clapping to hear how said only in driving operation and under partial load. if I give light gas at 50kmh in the 4th walk, it is especially good to hear. It is a x20se (8V) motor installed. oil change was made a short time ago(10w40) the previous owner drove with oil addition from Mathy, while always only the filter was changed never the oil. after the purchase the product was let out immediately. have also thought of pleuel bearings, but they should also make themselves noticeable in the empty run especially if one with the speeds spie I? maybe one of you has a tip? mfg