Don't know, but it was the darnedest thing. 290 IAS nothing... took it up a little higher, 320 IAS and the cockpit started to go up and down at a medium frequency. We were ****ed we got asked to do it, but we were glad we got to verify it for others sake. We almost went back saying there was none.
Didn't know what was causing it. A week later I asked one of the mechanics about it, he said it turned out to be a damaged rib. This was back when we had the CLT base.