Could be a number of things. Belts, pulleys, PTO and on and on. Your gonna have to check it out closer and see where it's coming from.I have a 1541 that only made it to 600 hours before i replaced the engine.
Now it has a chirping sound when starting or when i mow the lawn
This engine has approx 250 hours on it so i am hoping it is not on its way out.
If it's doing it under increasing load or when starting it cold, it could be a leaking headgasket..