Through further experimentation and other feedback on the Apple community I think I have figured out the common issue and HAVE A TEMPORARY FIX until Apple resolve this.
I have been experiencing the issue 2 hours+ into a bike ride. Another user mentioned moisture, and they experience it after a shower – that got me thinking. The new silicone/foam ear tips create an excellent insulated seal between the hot moist inner ear canal and the external dry / colder air. This is especially true for those on Plane flights where the airconditioned cabin is dry and generally colder than the temperature within in the ear. This temperature difference, moisture on one side and excellent seal with hydrophobic coatings is the perfect trap for condensation to form.
There are two layers of mesh together – one black on the AirPod and one white on the Air tip. My hypothesis is that water vapour is hitting this colder mesh surface and forming microscopic condensation which is trapped between the two mesh layers and interfering with the sensors that are driving the ANC systems.
I experimented on a 5-hour bike ride in the weekend. After 2 hours the noise started so I removed the air tip, wiped the black mesh part of the AirPod, with a microfibre cloth dried inside my ear with another cloth, waved them around in the air to dry out, put the ear tip back on and the issue went away! I followed this procedure every hour for the remainder of by bike ride and this fixed the problem! Not ideal but at least it is a temporary fix until Apple figures this out…
Others have also reported rotating the ear tip 180 degrees provide a fix – I think this is because in doing so they have released the moisture / dried the mesh which may have been minor at that stage – nothing to do with the rotation, but rather just the act of removing / drying and putting back on...