I suspect the torque converter seal that is known to fail on these cars may be leaking on my 2004 CTT or is begining to, but I'm interested to hear from others who experienced this how it manifested?

I have quite a pronounced whine coming from what I believe to be the gearbox which increases relative to engine RPMs. It isn't noticeable below about 3k rpm but after that it gets louder and louder and masks the sound of the engine.
While there is no fluid dropping onto the ground below the car, if you look behind the plastic covers or take them off you can see quite a lot of dampness from the bottom of the bell housing, and the fluid looks reddish so I'm betting it's ATF.

What other symptoms could I expect from being low on ATF? Will the gearbox put itself into limp mode if it begins to overheat?
The car is quite high mileage (over 200,000 miles) so internal wear on the gearbox is probably also a possibility.

Should I try any ATF additives and top it up and see how long it lasts before it gets low again?
Since the car has such high mileage I would probably put in a used gearbox with less miles if the current one needed to come out to fix this problem.

Any advice appreciated.
TIA