Hissing like a dump valve

[Brucielee]

OK so here's another strange one, 

2.5tdi a4 b6, when I accelerate it sounds like a v6 should, but when I let off the accelerator to change gear it sounds like a dump valve hiss, as far as I was aware there aren't any on diesels, it's not lacking power and there are no lights or fault codes to tell me the boost pressure is down, has anyone came across this before, maybe it's meant to sound that way, I don't know, just like to find out what it is incase it's a growing fault somewhere, although it sounds good 

Thanks in advance 

[Trevor]

Hi BrucieLee

Sounds like a split in one of the boost hoses causing the sound.

Maybe worth waiting until it becomes louder and puts a lamp on, especially if it sounds good 

Cheers   Trevor