Hi all, I'm new to this forum and have a question about the S1 evo2 versions of Audi's old Grp. B rally cars. The questions is regarding the chirping squeaking sound on the over-run or between shifts from the excess boost? In these cars what exactly causes this sound? I believe this to be the type of diverter valve used venting to atmosphere but am not convinced. I have also been told that they ran twin wastegates and this is the cause. Does anyone know this for sure? It can clearly be heard during much of the footage of this car in action although not all versions sounded this way. If anyone can help I would appreciate it. Thanks.