It makes the same noise with or without the CO2 turned on, so it seems to be the natural noise of the rotor. Why it chugs I don't know. Thanks for the recommendations.

Incidentally, I discovered today why it is necessary not to raise the CO2 ppm too high. I lost 5 fish and all the others were gasping at the surface. I guess that means the 50 ppm I measured was pretty close to the real value. I shut off the CO2 and the remaining fish recovered well, but the plants kept pearling like crazy for another two hours, before I turned off the lights. Tomorrow I will start with a much reduced CO2 flow.