That's a perfectly normal speed test. You may be hearing the drive spin down and up at the zone changes, which is normal and is depicted in the graph where the speed changes.
the drive may well sometimes slow down during a burn in response to the media not handling the speed, but your graph does not show this happening.
Depending on the media and speed settings, a drive may also be spinning slower as the burn progresses, even though the linear speed is increasing or staying constant. This is typical for Z-CLV burning and also is normal. As you see by the yellow line in the graph (RPM), the drive is slowing in each zone to maintain a constant linear speed.
Z-CLV burning is common for CDR's in DVD burners, most CDRW burners run at CAV mode which is a constant spindle speed (RPM) and is quicker but more difficult to maintain good burn quality.