Howdy fellas

Just wanted to pick yours brains. I have a Milltek exhaust full system and I've been hearing a slight blowing from somewhere. It only happens when I'm braking so it is odd. I've had blowing on an exhaust on another car which was more obvious but this is puzzling me. My mechanic is away on hols atm so just waiting for him to get back but was hoping maybe someone had experienced this before. I thought Milltek's were stainless steel throughout but might be wrong. Any ideas?