When I purchased my XF it came equipped with Pirellis and the passenger front tire had a bubble which I drove around with for several months with no problem. A coworker a mine just told me a couple of days ago that his girlfriend had a bubble on one of her tires and he told he told her not to worry and mentioned my situation and guess what....that's right, she had a blow out. Luckily, she was not driving on the highway nor going fast so the moral of the story is don't drive around for too long with a bubble on the tire because you never know. I ended up purchasing some General Exclaims and have been very happy ever since.

Good luck and hopefully you can resolve your problem.