If you have the Michelin Energy Saver 225/50R17 tires be on the lookout for bubbles or bulges in the sidewall. If you continue to drive with the bubble, the defect could open into a crack which deflates the tire. Evidently this is a known issue with these tires. My tire has 12k miles on it and has to be replaced. The guy at Discount Tire says this is caused by hitting a pot hole or curb, but I have never seen other tires do this. Maybe the Energy Savers are more prone to this type of damage? I also found this website with numerous complaints.
Michelin Energy Saver A/S Vehicle Tire Consumer Complaints