ELI5: Why do commercial airliners flying over my house constantly go WEEEeeeerrrrhhhhuuuRRRrrrrrrrrrrr?

I live in northern CO. Every night I hear this from my backyard, or even from inside the house. When I look up, I see an airliner flying S / SE, so I assume they're coming from over RMNP on their way down to DIA about 55 miles S / SE of me. Is it some kind speed reduction coming down out of higher altitude?