Yes that is how the fan on the new HX750 works. If/when your system reaches a certain thermal/power threshold the fan will turn on again.
A lot of people are confused by this behaviour. Corsair thought this type of behaviour would be less confusing than the fan only turning on when it has reached a certain thermal/power threshold.
Your PSU runs (normally) in a semi-fanless mode. The fan does the little movement at first simply to let you know it (the PSU) is on. When heat builds up, or your power draw on the PSU draws enough heat, the fan will come on only as long as it needs to in order to cool the PSU components down. It is not something to be concerned about, in fact it is a feature found only in better power supplies