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I have seen the transmission pump start to make noise when they are low on fluid. I would suggest having the transmission fluid level checked with a special tool at a shop since there is no dipstick for it. You could purchase the special tool on your own here. http://www.amazon.com/Assenmacher-M7226-Checking-Levels-Mercedes/dp/B001PCGXRQ/ref=sr_1_6?ie=UTF8&qid=1446920028&sr=8-6&keywords=mercedes+trans+dipstick
The low fluid can cause the engine to intermittently rev high because of the transmission slipping. If fluid level is ok, you could have a failing front transmission pump which would be indicative of the high pitched noise. I would suggest at your mileage that a rebuilt transmission would be a better option than trying to repair yours.
Please let me know if you have any questions before you decide to rate me, thanks!