When I drained and fitted a new filter earlier this year....and then a couple of months ago when I did my Gearbox Oil Cooler...the fluid looked fine and the sump magnets were relatively clean looking (for tens years old) - see my How To's in the How To section for photos.
Never have found any reason for it as yet....it is as if when the 'box changes down a gear when you apply the brakes or the Lock Up Clutch disengages its clunks....
I have no vibration, judder, whine or other noises...just this clunk when you first apply the brakes ...