I would check the belt tension and maybe the bevel gearbox oil, with that mileage and age you may have "the gunge" in the BB rather than oil.
As for the so-called "gunge" this will not contribute to the very disimilar "chirping sound" from the belt/sprocket interface.
However, when you have to go the dealer or mechanic to swap out the sprocket and tweak the rear end it is a most perfect time to change gear oil. Mine was done after about 3,500 miles when my first sprocket bit it. My dealer did it without asking. When I go in again, it will be done again at the 16,000 mile mark. I will ask for this. It still whines. It is directly attached to the frame. That is all. This is a most vocal car!