A squealing noise coming from your engine compartment may be an indicator that belt replacement is imminent. Items usually powered by the serpentine belt are the power steering pump, the air conditioning compressor, the alternator and the water pump. The squealing may temporarily subside on wet days, but it will probably only get worst over time. It is not a good idea to let the belt go until it breaks, or you could lose power to some or all of your engine's accessory systems.
Open the hood and locate the belt routing diagram sticker on the radiator cross member or fan shroud. Check the owner's manual if your vehicle does not have a routing sticker. Draw a diagram of the belt routing if the existing belt is still in place, there is no routing sticker and the owner's manual is of no help. Use a repair manual to obtain the proper belt routing, if necessary.
Locate the spring-loaded idler pulley at the front of the engine. Note that most idlers have an exposed pulley and a cylindrical housing that encases the spring. Push a 6-inch extension onto a 3/8 inch drive ratchet. Push the square end of the extension into the square hole in the idler pulley housing. Rotate the wrench handle in a clockwise direction to increase the spring tension and move idler pulley away from the belt. Hold the wrench in position and slip the belt off of the closest accessory pulley to you with the other hand. Release the wrench handle slowly until the idler is no longer under tension. Skip this step if the serpentine belt is broken and the idler pulley is not under tension.
Slip the belt from the remaining accessory pulleys and remove it from the engine bay. Unwind the belt from the pulleys and surrounding engine parts and remove it if it broke while the engine was still running.
Hold the edge of a metal straight edge to the ends of the accessory and idler pulleys to ensure they are all in alignment with each other. Note that one pulley out of alignment will cause a replacement belt to fail prematurely. Replace any component that may have worn bearings causing its corresponding pulley to be out of alignment. Spin pulleys and listen for squeaking or feel for roughness to see if the bearings are worn. Note that an idler pulley with a worn tension spring may cause its pulley to be out of alignment. Unbolt the idler to remove it and replace it with a new one, if necessary. Bend or shim accessory brackets with flat washers to bring the pulleys back in alignment, if necessary.
Slip the replacement serpentine belt over and around all but the closest accessory pulley to the idler pulley. Use the routing information on the routing sticker, the owner's manual, your notes or a repair manual to ensure the belt is routed around all pulleys correctly.
Increase the tension on the idler pulley with the ratchet wrench and extension. Slip the belt over the last pulley. Release the wrench slowly and remove it. Note that a small pointer on the idler housing should be within the two marks on the idler arm if it has the proper tension. Replace the idler if it does not have the correct tension.
