Grinding gearshifter noise

[ronw123w124]

Hello everyone, I'm a newbie here. I'm having a problem with my w126 260SE. Took it out for a spin from the shop, as I was heading back to the shop I accidentally flipped the gearbox from D to 3, then I noticed that there is a grinding coming from the gearbox when the car is in motion, but when I flipped it back to D everything returned to normal. Even the mechanic who's working on the car found it strange because the transmission was working perfectly, the shifts are smooth and seamless, I hope this problem would'nt set me back by a fortune again