Pneumatic noise in 737 cockpit below 4000'
One thing that for a long time has haunted me is the noise that you have in the cockpit below 4000ft approximately. It resembles the sound of pneumatic friction in a tube and also a running tap.
I've tried to figure it out by asking our captains and searching in the FCOM but now I have to face it and ask here. One captain told me it has something to do with a specific valve opening and closing. I've kept an eye on the ram door lights but they don't seem to have any correlation to this noise.