If your vehicle is showing fluttering or 'turkey' noise when letting off the gas pedal, here are the most likely causes and the parts you'll need to fix each one.
Installing a cold air intake removes the factory sound deadening, making normal turbo bypass noises highly audible. Additionally, an aftermarket blow-off valve with a spring that is too stiff will intentionally or unintentionally cause this 'turkey' flutter (compressor surge).
The factory bypass valve releases excess boost pressure when you let off the gas. If its internal diaphragm tears or the valve sticks closed, the pressurized air forces its way backward through the turbocharger compressor, creating the fluttering noise.
The blow-off valve relies on engine vacuum to pull it open when the throttle closes. A cracked, loose, or disconnected vacuum hose will leave the valve shut, causing the air to back up and flutter against the turbo.
If the wastegate actuator spring weakens or the diaphragm leaks, the wastegate flapper can flutter against its seat as exhaust gas pulses past it, which can sometimes mimic the sound of compressor surge.
Severe, prolonged compressor surge can destroy the turbocharger's thrust bearings. If the bearings fail, the compressor wheel can wobble and chop the air or scrape the housing, creating loud fluttering or grinding noises.