Ya I had that too despite thinking everything was good and clear underneath. I ended up finding that the O2 sensors were actually touching part of the heat shield underneath and a couple places of the heat shield were really close to the pipe. I used a rubber mallet to bend the heat shield and give more clearence in any close areas and made sure each O2 sensor and attached wiring were not brushing on metal.
Don't think this is a problem with test pipes but with a full x-pipe the black cross member brace was also vibrating and had to slightly modify that with a couple washers to give a bit more clearence.
But go under and pound the heat shield to give more room everywhere. Once you have it correct there is NO vibration.
The vibration really took the fun out of hte mod for me. It was super annoying and felt like crap whereas now without it I love the mod.