If you run at a faster idle for a while to let the engine internals cool down at bit, does the chirping still occur when you throttle back?? Probably low oil pressure, so its time to check the oil. If the dipstick is low, add some. If it is high, you may have a seal in the injection pump which is leaking diesel into the oil, cutting its viscosity. An oil change, perhaps with a higher weight oil, may solve the problem, but if it reoccurs you may need a mechanic.