shango

I have a w202 c200 1995, i suddenly started hearing a low whining/ whistling sound coming from the transmission about a week ago when i put it in Reverse from P or from N the sound lasts for about 2 seconds and goes away.When you start it up cold and engage "r" its silent and works nomal but once the car warms up and you try to engage "R" you hear the sound. I completely drained the transmission and torque converter and replaced the filter and fluid but this didnt help. It goes into gear fine, drive and reverse and it changes very smoothly the only problem is this low whistling sound
when i angage "R". Anyone have any ideas what could be causing this?