Just wondering, I went and bought the C-systems 750 mark II pump, and it's running fine, one thing I wonder though is right now when it's leak testing, I can hear the motor noise change. This change happens every 5-15 seconds, and occurs for about a second, and it sounds like the load of the motor is higher, as in the motor sounds more burdened during that second. Is this normal? If not, what's going on?