car is a 1990 190E 2.6 auto with about 100k.

I've noticed this for sometimes now, probably back a year, but now have taken considerable notice of it. here is the deal. the car will creak when I shift into R or into D, and sometimes from P to D the car will make a "push" forward with an audible sound. the creaking sound is like metal to metal (like creaky door hinges), and it's more noticeable in the morning and especially when shifting to R. the sound doesn't seem to happen often after the car has been driven or "warmed." do I need to replace the flex disk? how much will that service cost? thanks.