Compressor surge or diverter flutter?

[arronleeds]

Just noticed today that when I’m in low gears, like 1-3 and the car automatically changes gears and I then let off the throttle I hear sort of a single flutter sound, is this normal? The car has been remapped but I’ve literally only just noticed it today.

 

[Dominic]

As far as I understand it, the fluttering sound is often the noise made when compressed air in the inlet rushes back through the turbo when you left off the throttle and the engine isn't under load, diesels tend to do it since most don't have plates in the inlet or BOV/diverter valves. Apparently normal and is catered for in standard design tolerances, it's only an issue if it does it whilst you're on the throttle, that means you've got bigger issues.

But that's just what I've read and I'm no engineer.