46RH making noise in Park and Neutral

Hello all

The noise is coming from the bellhousing area. Sound is rotational and hard to describe. Growling, rumbling. The fluid level is spot on, no other noises, and the transmission works as advertised. The noise subsides in any forward gear and reverse. Last time I dropped the pan, the fluid was red and clear with no big chunks. I dread the thought of pulling the transmission.