We report an experimental study of low-frequency noise (LFN) in n-type tri-gate (TG) Si nanowire (NW) field-effect transistors (FET) with cross-section width down to 10 nm. Drain current noise spectral density has been measured in all ranges of transistor operation. The LFN behavior shows good agreement with carrier number fluctuations with correlated mobility fluctuations even in ultra-scaled TGNW. We did not observe large contribution due to surface orientation difference between the (100) top and (110) side-walls of TGNW. Moreover, the influence of drain voltage and extracted oxide trap density are roughly the same for reference wide devices and TGNW FETs.