93 Delta 88 Royal with a washboard rumble in the Transmission.

We noticed a rumbling feeling (like rolling over wake-up strips on the highway) when the car would shift into higher gears (maybe overdrive) we are talking around 45 to 65 MPH. But then it will go away and run smooth. Kind of like it has a problem getting into gear but once there it’s ok until she has to shift again.

Oil, Oil Filter chged reg + or – a few hundred @ 5 K increments.
Last tune up, 139K = Air Filter, Fuel Filter, Wires, Plugs, cleaned injectors and changed the trans filter and fluid.
When we first noticed the problem (145K) I discovered the trans fluid was low (just on the stick) Trying to determine where it went, we found it in the Radiator overflow tank!
I replaced the radiator and fluids. The tranny fluid now stays in the tranny but the rumble is still there. I added two pints of tranny aid and the problem seems to be better but not gone.
What do ya think?
I know, what do I want for 145 K. Hey, she has a couple of air leaks but gets 30 MPG and rides better that any car I’ve ever had. AND still has power.

it sounds like you are having a torque convertor problem check your local tranny shop. good luck