I replaced a bad Idler pulley and all was well for about a month. Then, this "sh sh sh/ch ch ch" sound came out of the blue. I replaced the belt and checked the pulleys, which all turned quietly by hand with no excess play. The speed of the sound is about 432 chirps per minute at 720 RPM and gets faster with acceleration. I re-torqued the exhaust manifold and, even though it needed it, there was no change in sound. Any ideas? Thanks.