Just want to pile on here to say that this solved the squeaking noise for me. It seems counter intuitive to apply grease to the outside of a pulley, or to a belt, but it did the trick.
Been using an X1C for over a year now, and suddenly developed this squeak. Thought for sure it was a cracked/broken ACME nut or bearing, but no… it was the idler tensioner pulley like others here. A half a drop of quality oil (not WD-40, that’ll evaporate in a week!) just above the metal roller (from the side, not from where the M2 screw is) will work its way into the friction area and silence it.
My guess is, any factory-applied grease dries out after awhile and it starts squeaking. Probably a good idea to consider this idler pulley assembly a consumable part, with a lifespan of a year or two, depending on usage.
I had the same issue, it was the Z-Belt under the machine (P1S). I got the lube pack that comes with the printer and put some on my finger and ran it along the lenght of the belt that was exposed, trying to only coat the top edge rather than the teeth. A bit fiddly but as the printer moves the belt I figured it woudl work the lube into the bearings or wherever it was squeaking. Works like a charm, no squeak now.