I don't know why your whistle happens at a particular time. I don't because you didn't bring it to me (bg).
If my conclusions are correct, monitoring the system will provide the answers. Is the valve too open or too closed? Why? Does the system do its job? Both during the event and at other times?
This is an "idle control system". Does it?
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician