The idler pulley bearing on the '97 has been noisy at idle from time to time for a while now but became consistently noisy on Friday.
Contrary to (some) popular opinion, the idler wheel appeared to be the same as the wheel on our '94 3.9 Disco.
Since I had an extra idler pulley bearing on the shelf, I thought I'd pop the pulley off the 90 and take a closer look.
Sure enough, it's the same one as the Disco!
Now, the little bearing inside the pulley / wheel / whatever you want to call it is the culprit. Makes noise when I spin it and even wobbles a little.
The Disco was an easy fix, the retaining clip had "ears" that allowed removal of what you would expect to be a circlip with a pair of small flat screwdrivers.
No such luck with the 90
The ends of the retaining clip appear to be intentionally beveled to make removal difficult, but the sale of a complete new pulley quite simple.
After a few moments of head scratching, I occured to me that there was enough material exposed on the retainer to drill a pair of teeny (turns out 1/16th inch works well) holes .... simple to remove the retainer - now an official circlip - with your garden variety circlip pliers.
All is well, now all I need to do is restock my inventory with a bearing and a new serpentine belt ... also the same part as the '94 Disco.