Barudan Whistling Sound

Hello Everybody
I would like to know what causes an embroidery machine to produce a whistling sound. I have a barudan machine. Although the sound is coming every few seconds, and on certain stitches, the embroidery is still perfect. So If anybody can tell me what's going on, please tell me, because I just called a tech and the noise started coming after he left, and I don't feel like paying like $300 dollars. Also, I've lubricated the whole machine.

Please Help