Another 205 owner here mines a wrc so has all that anti lag bollox on manifold if that makes any difference the noise you are reffering to is compressor stall when you let of throttle pressurised air is forced backwards though the compressor housing a dump valve prevents this by releasing excess pressure, mine still neighs abit but mine is not realy standard engine wise, also check your front suspension for wear there excelent when well maintained but expensive when not, i know of some st205s being broke if theres anything your after