Peaks from 70 – 90 bp (see the trace below) are primer dimers from the 2nd PCR and result from incomplete removal of small molecular materials during the final magnetic bead purification. These are usually caused by inaccurate pipetting of magnetic beads when working with a large number of samples at once OR insufficient removal of supernatant  and ethanol during washing steps. If these peaks are significant, one can pool all indexed libraries (that will be sequenced in the same lane) and perform one additional round of 1.2X magnetic bead purification, before the quantification step prior to sequencing.