" its due a transmission oil change now so you kill two birds with one stone."

I would prefix this " in my opinion" but there is no recomendation in any service bulletin to change the gearbox oil.MB service cars that have done upto 1million kms without changing the gearbox oil.

3 year oil change? there's nothing in the service bulletins to suggest that and MB boxes last a million km's, daylight robbery i tell ya

...;))

As Gordonbennet says, first thing to usually do with an MB box when you have a whine etc, is to get the fluid level checked/changed. It is critical that the correct grade of fluid (meeting the MB spec) is used, plus there's a set procedure to follow for checking the fluid level, so best to take it to a garage who know about Mercedes.

OP doesn't state if this is the older 5G or 7G box. MB changed the guidelines on the 5G boxes from 'sealed-for-life' to change once at 37,000miles. Many MB indys (and main dealers) recommend to carry on changing the fluid (plus filter) at approx 37K intervals - I certainly stick with this. If it is the newer 7G box, not sure what the guidelines on that are.

Which ever gearbox it is, get it checked out sooner rather than later!