Sounds exactly what I went thru with my 727 tranny. I heard a loud whinny noise only during certain RPM's of the engine. Its very hard to track down engine noises but I thought it was coming from the tranny. I would put it in gear, hold my foot on the brake while in drive and I heard the noise. Put it back into park and the noise stopped. Then one day noise seemed to be coming from PCV valve. I replaced the PCV valve and my whinny noises went away. Try eliminating the pcv valve while your testing or if you have another laying on the shelve, just try trading it out. Let me know.