Think you are hearing a bleed shift, when an engine switches between high-stage and low-stage bleed regimes.
Now I know that there are various 'bleeds' from different points along the compressor stages, for air conditioning and even starting 'the other' engine plus, I imagine, other services, but your answer both confuses and intrigues me.
I am fairly certain the noise cannot be control surfaces being moved - their cycle time is too long for one thing - but changes in relative pressures being vented to atmosphere sounds promising - so to speak.
Would you elaborate for this non aero engineer please?
Roger.