neddie

Hi I have bought my first Yamaha after a long spell with Kawasaki. I have a YZF1000R Thunderace 1999 It has just developed a strange noise/squeak. When idling it's fine but as soon as you rev it squeaks for a second or two and then goes away but it does it everytime you rev fromm idle. It sounds like it is coming from the exhaust. Has anyone else heard of this? Thanks