Laptop making whistling (buzzing) sound!

I bought a new or supposed to be new laptop dell inspiron 5457 model yesterday. Now when i plugged in my earphones it is making buzzing noise from speakers and my earphones are not working on it. I checked my earphones on other devices and they are working fine with them.

Suposed to be a new laptop because the DELL guy gave me a display model by repackaging and I figured this out when i unboxed this model after bringing it home. I am sure he gave me the same demo model because from past two days i was in contact with him at his shop and use to check the demo model thouroughly and found out same BOTTOM FAN GRILL DIRT on my laptop at same place. 

When I was taking the delivery I enquired about the demo model which was not present there and when i asked him he politely told that he has sent back the model to company, while other models were at the same positions. He was constantly saying me that all the demo models keep on changing . So they put new models.

Now this buzzing sound is problem, no headphones are working and i want a complete replacement of my product