Brake booster suction sound after depressing brakes

[ghetdjc320]

Have an odd little vacuum sound. I run a dual diaphragm 8” booster and an electric vacuum pump that runs fairly high vacuum (24in). Recently, when I let off the brake pedal their is a vacuum suction sound that lasts for just a second then sounds like a valve closes shut. Almost sounds like something is a bit sticky in the booster and needs some oil. The booster isn’t leaking at all while breaking and works fine otherwise. Any ideas what it could be?

[Pete M]

vacuum escaping past a seal?  I've never dealt with a system that runs that high.