TL;DR
If you can use iotop, do so. Else this might help.
Use top, then use these shortcuts:
d 1 = set refresh time from 3 to 1 second
1 = show stats for each cpu, not cumulated
This has to show values > 1.0 wa for at least one core - if there are no diskwaits, there is simply no IO load and no need to look further. Significant loads usually start > 15.0 wa.
x = highlight current sort column
< and > = change sort column
R = reverse sort order
Chose 'S', the process status column. Reverse the sort order so the 'R' (running) processes are shown on top. If you can spot 'D' processes (waiting for disk), you have an indicator what your culprit might be.