Whistling Noise Heard When Turning Off Cold Water

[jkon]

I live in a ranch house that I purchased newly built 28 years ago. The house has oil forced hot water heat. When I turn off the cold water at both my kitchen sink or bathroom sink, I hear a whistling sound that lasts about two seconds. Sounds like a whistling noise that a bomb makes when dropping through the air. It doesn't always do it which makes it difficult to isolate where it is coming from. It is not my toilet flush valve because I removed the tank lid and listened and it is not coming from the toilet. The whistling just started about two weeks ago. Any ideas of what it may be and how to isolate it would be appreciated. Thank you all!