Rattling noise.....Please help!!!!

i just had a custom 2.5" off road H-pipe with a pair of original 2 chambered flowmasters installed on my 03 GT. The pipe runs from the headers all the way to the mufflers. When i drive the car and the rpm hits the 2000 rpm range it makes a rattling noise ...in first 3 gears its not a big deal but when on 4th or 5th gear its very annoying because i stay around the 2000 rpm range. I thought it was old catty shields that i told the guy to weld around the new pipe that were making noise but when i went there, the guy took out the O2 sensor and was hitting it with his fingers and it was making the same noise. So my question is did this happen to any of you guys? is there something wrong with my O2 sensor that its making this noise or are all of them like that? what do u guys suggest? By the way the noise is coming from inside the O2 sensor as if something inside is loose? Its the sensors near the mufflers not the ones near the header. what should i do please help my 2003 mustang sounds like a 1991 civic with ractive mufflers now

none of u guys have a clue what it might be??