That was the first and most annoying rattle in my car. Took weeks for them to find out where it was coming from,never mind how to fix it. They eventually asked me to take a mechanic out and replicate the sound. Christ knows how they didn't manage,because the first patch of rough tarmac had it rattling away. The mechanic put his finger on the handbrake button and it stopped. He took it into the workshop,"packed out the button" (whatever that entails) and it was fixed. It's never rattled since. One less to worry about.