Hi Carl. I ran a thread on this about 18 months ago or abouts.
It's only the Diesels that do it. Maybe because they have a separate vacuum pump. Mine does it and so do quite a few if not all
Once run a few seconds all is well. I've never had a problem with the brakes. Have you noticed too that if you sit with your foot on the brake pedal it will slowly sink to the floor?
Again never had an issue with that either. Just erm... Weird
I noticed that if I pumped out the vacuum, as it filled, I would also get a low tone coming from somewhere. Goes as soon as pressure returns.
Strange.
Regards. Steve.