Problem with the fill valve!!
Last week the Van Nuys Plumber who came to our house was really a proficient one. There was a strange whistle sound that was coming from the plumbing system since few days and I was unable to find the exact source for this noise. A quick look on the local directory found me the number of this plumber and I immediately called him up and explained to him my problem. He came to my house pretty soon and started on with finding the toilet that is causing this problem. He soon found the toilet whose fill valve was slowly leaking and was causing the sound. He repaired the fill valve and the noise completely stopped.