The transmission can be rebuilt at a good (MD qualified) shop for less than $3000 total (R&R etc.( with a warranty better that the MBZ one. It 'sounds' like a bearing on the mainshaft or the internal torque convertor (scuse me --fluid coupling) is gone. A grinding sould that changes with car road speed is on the output shaft. One that changes with engine speed only is on the input shaft. One that changes with TQ slippage (like in D at a stop) is the TQ internal bearing. 'Some' whirring noise that fades away is 'normal' when shifting from D to N or P when at a stop light/sign. This is merely the TQ output turbine speeding up to match the input(engine) side and the internal bearing will whirr. And the input mainshaft of the tranny will also accelerate from 0 rpm to engine speed when gear lever is moved to N or P from D.