I went out to a Golf on saturday with this same fault and it was a sticking flap as mentioned. I think it's also called an anti-shudder flap as it closes at engine shut down. His neighbour said his timing belt must have snapped and with his constant engine cranking eventually it was displaying the red oil pressure warning in the instrument cluster and beeping at him. He was thinking a new engine was required . I got it running and took it back to the workshop to remove the housing and clean it thoroughly. From memory they aren't a ridiculous price for a new one but this was saturday afternoon. I always remove that boost pipe during a service to remove build up before it gets to the stage that yours is probably at.

Liam