Tire loses air when intake valve is at 6:00 position?

I have had a slow leak on my rear driver side tire. In the winter, it's a pretty constant drop to about 12 PSI. I inflate it to 36psi, but it drops steadily to 12 PSI. In the warmer, summer months, the drop is not nearly as constant. I replaced the tires with all brand-new tires. That tire still experiences the same drop. I have noticed when parked in the garage, if the air intake valve is at the 6:00 position of the tire, there is a hissing sound. As I mentioned, the tire is brand new. Is there an easy remedy?