iphone 4s: piercing noise cuts in whilst on a call?

My first iphone 4s which I got at the end of March did this from day one. Whilst on a call, a loud shrill tone would cut in and it was so loud I had to take the phone away from my ear; it lasted up to 20 seconds then the call would cut back in again. On the other end, the person I was speaking to wasn't aware of anything, no noise but it went kind of dead for the duration. This problem was intermittent, not on every call, sometimes twice in one call.

I took the phone back to apple who exchanged it straight away for a new handset which was brilliant service. The very first call I made on the new phone-bang, exactly the same thing. I could not believe it. I got to wondering if it may be the brushed steel protective case so I changed that for a plastic one but it still does it. Not as often as the first phone but it still does it. Any ideas what it could be? I am almost embarrassed to go back to apple again but it can't be me can it? The phone is only 2 weeks old!