115 tranny grind

[67fintail230s]

Hello,
My 75 230.4 automatic has developed a transmission grind as it goes into drive. Once it is in drive, it shifts through the gears nicely & there is no noise going into reverse. If the idle is to high it takes longer to go in. I have replaced the filter and fluid (Dexron III/Mercon) to no avail. Is this an adjustment, vacuum modulator, or am I due for a rebuild?

Thanks in advance

Anthony