Had the same problem with my '84 244. If you pry the round switch plate for the overdrive out from the top of the shifter, you can see a circular piece of thin foam rubber with a square cut for the switch. This foam rubber is to dampen vibration and keep the switch from rattling. When it get worn it doesn't work. I cut a new piece of foam and it's quiet now. I can drive without holding the shifter again.

I have a solution similar to Dave's - except I used "rope caulk" under the shift-pattern plate thingy. Dave's paper solution may be better, though, because with caulk you run the possibility of (the caulk) interferring with the OD push-button switch.

I used a couple of small pieces of paper under the plastic disk that has the shift pattern. The paper raised the plastic disk with enough pressure to stop the noise.