However, upon investigating a strange rattling noise that I was hearing on the opposite side of the printer, I found this:
It seems the torsion exerted on that square projected section holding the bolt that supports the bed was too much for the plastic, after the fatigue left by printing a few kilograms of plastic over 6 months, especially with the heavy 5x230x230mm aluminium plate being supported by the corner bolts.
It's an uncommon failure mode - many FFF 3D-printed things can break if they are subject to a stress that tends to peel layers apart. Normally they should be fine with a reasonable stress acting in the X-Y plane that the object was printed, but here the sharp corner at the bottom of the bolt support has probably caused the stress to concentrate there.
It looks like I'll have to glue the part back together and print a new one, since funnily enough with all the parts I've been printing for the Prusa Mendel, I haven't made spares for such structural parts on my own printer, not knowing that this part would break so soon, having forgotten any worries I had about the part when first assembling the Y-axis.