I have a new (to me) '88 740 Turbo with low miles (193K) that has been running smoothly with a couple of minor exceptions.

Starting yesterday, the car began "growling" while accelerating. It typically is at its worst at about 2400rpm, then seems to quiet down. I can hear the growl quite plainly while in the car, but it is not noticeable with my head under the hood or under the car.

I initially thought it might be transmission related as the car hard-shifts between 2nd and 3rd and the fluid is just barely above the "Min" mark, but it growls even in "Park". With regard to the ATF, where can you get Dexron IID? Is there something else (simple) that could cause the hard shift?

Thanks