Hi,
I am facing the exact same issue with the constant knocking sound, but with DS-2CD2412F-IW. Were you able to resolve it finally?
The strange part is that it worked fine for almost an year and then one fine day, I started getting this knocking sound. The firmware was always at 5.1.2 and I hadn't touched its settings or upgraded the firmware all this while.
However, since I started getting this noise, I have tried updating the firmware to the latest 5.3.0 (5.1.2 -> 5.2.0 -> 5.3.0) and tried all available audio encodings, but to no avail. I get this knocking sound in both iVMS-4500 iPhone app and when viewing via WebComponents/Quicktime plugin in Safari on Mac OS X.
I also tried to change the location of the camera to check if it was the wind noise or due to electrical disturbance from the particular socket it was plugged in to, but the result was the same. I bought the camera from a Hikvision authorized vendor.
Can someone please help?
Thanks!