Ballistic

I was enjoying the weather and my car today. I was a great day for a drive and I was motoring along in the S' powerband. Around one of the many sweepers, I downshifted (practicing toe-heel if that is important) and heard a squeak as I am engaging my clutch. This sounded very much like a turbo pressure release valve going off and its only happened once (maybe twice. First time I didnt notice but passenger said he heard it). My question is? Has anybody else have had this thing happen? Should I be worried? And if anybody knows? What is it?