reading about whistling and creaking birdies made me wonder if there are very noisy folders and others that tend to keep quiet. What sounds do your folding bikes produce, can you track it down and stop it - or does it belong to the bike no matter what you do and do you perhaps like it (some love the sound of a Speedhub)?
My brommi sounds: "rattle rattle rattle" especially when going over rough terrain. I could not figure out what it is so far. I suppose it derives from the roller wheels and the chaintensioner or the mudguards.
My Carryme has always, not only when coasting, a clear but full hub-clicking "drrrrrrrrrrrr"- and is otherwise absolutely quiet.
Also can you say something about the components perhaps. Like some tires for example the Marathon supreme:"sing".