I'm guessing the knock you hear is the aircon compressor clutching in/out. When clutched in the fan also runs via an electrical switch/relay (at least below a certain road speed, I don't know about your car specifically, possibly it runs all the time the clutch is in). When the clutch releases (with a clonk) the compressor stops and the fan will also stop.
The compressor will run until a certain system pressure is reached, then clutch out and the system pressure will gradually drop as the refrigerant is expanded and cools. The hotter the ambient the faster the cycling tends to be and the more time the compressor needs to run.
It could be that your system is a bit low on refrigerant and is cycling too quickly, and needs a re-gas.
If it works acceptably at slightly lower ambient temp then you may decide to live with it for a bit, otherwise see about a re-gas.
Just a guess from your description, I may be totally wrong.