I've found this car now which does sound similar (described by them, perhaps more aptly, as a "chuff"), has similar characteristics with no stated performance loss, and was a vacuum line:
https://youtu.be/r9B0CiirP54?t=280
I'll try and inspect all the hoses I can see more closely later in the week. I'm still waiting to hopefully get better parking soon which would give me more opportunity to investigate things.