I've experienced this with nearly all of my cars, not just the Mustang, when it gets cold out. I heard a squeaking noise going over bumps or in dips. I only notice it when the radio is off but I am sure it's nothing to worry about. Did you just start noticing it?
Even V8s suffer from this squeaking shocks issue. Mine started at 5500K+ miles. Took it to dealership yesterday and they confirmed it to be bad rear shocks. New on order and it will be installed for free under warranty.