I had this exact same noise a little while ago in my R32 GTST... I did a full brake bleed with new fluid and it got rid of it for a while. But about a month later it came back again...
About 3 weeks ago I overhauled all callipers... Got new pads and Motul Dot 5.1 fluid, and havent heard it since. Hoping it wont come back
Cheers