A few weeks ago, we started to hear a continuous high-pitched whistle in our condo. Very annoying. After checking everything obvious, we started asking around and checking with Prof. Google.
It turned out to be the gas fireplace. Who knew?
According to the tech who fixed this in addition to doing a routine clean and inspect, a small piece of rust or grit can migrate into the orifice of the pilot. And guess what, rushing gas can create an audible whistle. He fixed it 5 minutes after walking in the door by blowing a jet of air onto the pilot orifice and then relighting the pilot. It wasn't new to him.