It's a hardware problem, but it doesn't occur with all 2nd gen pods! I had the 1st gen for a few years and now bought the 2nd gen with USB-C on Black Friday. Immediately after purchase I noticed a wind noise when cycling, as if someone was blowing into the microphones.
Since I noticed a crack in the housing of one of the pods, I went to Apple and had them both replaced. On the bike ride from the store to work, I noticed that the new pods no longer had the problem. The noise is the same as with the 1st gen, so you can hear wind, but it's just normal wind noise blowing around your ear, not a processed sound being replayed on the pods.
Before that I thought maybe the problem was due to the slightly different design (e.g. vent on top) and taped the vent as well. Didn't change anything. I bought the 2nd gen with USB-C. These were made in Vietnam. The replacement pods now come from China. The versions (A3047, A3048) of the pods still match Apple's specifications for the 2nd gen with USB-C.
So I can only advise everyone to go to Apple and exchange the fauly ones. The firmware version doesn't seem to matter either. The new ones were initially on 6A300 and now on 6B32. It makes no difference.