Morning fella, sounds like an awesome motor turbo selespeed; what kind of power you getting it to run do you know?
I am guessing you're experiencing a phenomenon known as turbo flutter when you come off throttle and is evident down the exhaust as a fluttering sound because the turbo has stalled.
As said above, a dump valve, or blow of valve as they're sometimes referred, is designed to release excess pressure between the compressor side of the turbo and the throttle body. The compressor obviously doesn't stop turning when you lift off the throttle but the throttle body will shut, so you're trying to compress additional air into a closed system, creating a spike in pressure. The dump valves works on a basic spring, just like the valve in your tyres really but to let pressure out not keep it in. When the pressure on the valve (built up inside your intake) exceeds the strength of the spring, the valve opens and pressure is released.
The problem is when the pressure builds up and isn't released the pressure builds also in the compressor, causing the turbo and compressor to slow down very quickly, as the back pressure acts on the compressor, so the benefit of having the pressure released allows the turbo to continue to spin and you'll in theory achieve less lag and spool times as a result.
There are two types or BOV, atmospheric and recirculating and both do what they say on the tin really. they either release the excess pressure to the atmosphere or divert it and recirculate the pressure.
As said above it depends on the setup of your system on which you need as it depends how the car is measuring the boost and what air is coming into the engine, as you don't want to be getting rid of air the engine believes is being delivered to it.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso