Iphone 4s making unusual sound out of "receiver"

I traded my android for my friends 4s yesterday, and it has this very strange noise when you make a call. He told me about this, and i thought it should be an easy fix. The sound goes away if the phone is on "speakerphone". And the strangest thing is that if you plug a headphone in, then remove it while calling, it sounds fine. However, the next call is back to the same. Has anybody else had this problem, and if so what is the solution?
Thanks