The noise disappeared as soon as revs increased above 1500.
Nothing looked awry, and a listening stick didn't pick up the alternator or the (new) compressor as the source.
So, whipped off the aux belt plastic cover and the belt, to find a rather dry-sounding main tensioner bearing.
. Without the belt around it, the hollow pulley could ring like a bell if tapped, and the bearings felt decidedly dry. Likely an original part.
The centre consisted of a thin pressed-in metal cover, with a narrow rubber lip on its outer edge.
Levering the disc out gave access to the outer bearing (I presume there is an identical inner one as well).
It's an open-sided ball-race, so fresh grease was pushed into it, with a spatula, with some extra encouragement from a conveniently snug sink plunger!
Immediately smoother and quieter, and some heavy oil dribbled down the other side towards the hidden rear bearing (optimistically).
Back on the car, the noise at idle is gone. Will have to wait for a hot run to see whether it lasts.
Good to see several of these new on Mister Auto, as well as the upper belt 'diverter' pulley.
A surprising (and welcome) range of BX aftermarket replacements on Mister Auto — one way of PSA/Stellantis dealing with NFP parts, while keeping it in-house. Thank goodness
