AUTO TRANS REVERSE GEAR NOSY

     Hi Everyone, need a little help, my S320 95' w/120,000 which runs perfect, is making gear sound in reverse (only). The gear noise reminds me of my old Ford Pick Up, but it lasted 200,000 miles. And the Mercedes is just a quiet gear noise. Its seem to me it's been doing this for some time, I just haven't notice.   I run Dextron III ATF, It's at the correct level on dip stick HOT. I change the fluid every 30,000 miles or so. I believe that's the correct fluid, I don't know of any other fluid Mercedes would run. Maybe I should change to 100% synthetic. So as anyone else notice a similar problem ?  Or would synthetic help ?    Thanks a million