The 722 repair manual actually references a light grinding or "rolling" noise in P or N, which is the front planetary gear set. It is considered normal to some extent.
It goes on to say if in doubt compare to a similar vehicle.

A similar noise in Reverse is attributed to excessive release clearance in the B3 clutch disk pack. This could be due to worn B-3 clutch disks.

In my experience, the rolling noise in neutral/park is normal; mine has noticeably done it for the last 100k miles at least - it now has about 230k.
It's not excessively loud, and cannot be heard if the heat or air is on low or a window is down, allowing other engine noise in.

Also, it seems on the 722 trans that the first thing that fails is the b-3 clutch pack - the fiber plates wear excessively, and eventually come apart. The first symptoms are a delay when shifting to reverse. Later, a clicking is heard when moving in Reverse, as the clutch basket moves so far back due to the worn plates that it strikes the housing. Time to get it repaired quick when that happens. Mine went at about 150k. Since I do my own repairs, i opted to pull the tranny and replace only the worn clutch plates; a pretty easy job as they are right at the front. Everything has been good for 80k since.

You mentioned there is no delay when shifting to R, so that might indicate no real problem with the b-3 plates yet.

As stated in the previous post, you should drain the fluid, pull the pan and look for debris or signs of problems. If nothing unusual is found, pour in new fluid and don't worry about it.

DG