Pump the brake pedal a few times whilst stationary and with the engine ticking over. Does the noise improve for a short time?
If it does it could be the vacuum non return valve which is located on the right hand side of the engine (looking towards the headlights) and is underneath the battery tray.
I had this with my 2011 D5 and it's not too bad a job to swap it yourself if you have some right angle torx keys.
The part (Volvo 31430964 - Brake Vacuum Pump repair valve) is circa £100 but it did the trick for me. Hope this helps...
Cheers
Robin
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