How to resolve whistling and hissing sounds in AirPods Pro 2 USB-C?

My son has a pair of AirPod pro 2 usb c, recently they have started to whistle when inserting and sometimes when using them and there is a faint hissing sound. I have cleaned them with the tools and fluids described in the instructions and took them to the Apple Store who said they were not faulty according to the diagnostic software, they suggested putting new tops on which we have but the problem persists and is getting worse with a rubbing sound when you move your head wearing them. Any idea what next step should be, they are just under 11 months old now.

[Re-Titled by Moderator]

Original Title: Whistling AirPods