Bell & Gosset Series 100 Whistling

My pump started whistling yesterday. I added more oil than needed in all 3 places but it's still doing it. Sometimes it stops for a few minutes, sometimes it continues non-stop. Sometimes it will alternate. The sound seems to come from the middle. Any idea what could be the problem? Listen by yourself: http://youtu.be/9agVRqPE0So Any help is welcome, thanks!