Is it in the direction that the scan head moves ? Then possibly a bad sensel or possibly dirt on the mirror/sensor head
If it is 90 degrees to the motion of the scan head, then 1) in the glass, 2) in the film, 3) electrical noise, 4) software/firmware. For 1, since this is a transparancy unit scan, try reversing the film (rotate 180 degrees, same orientation). For 2, if in film should follow the rotation, 3) scanner should be plugged into a electrical noise filter - UPS preferred, power bar, etc, for 4 try a trial version of Vuescan and see if it repeats.