I got the Prius a week ago Friday. On Saturday, i got a new set of tires, Goodyear Triple Tread 195/60/15, at Discount Tire. I told them about the sensors. He said it was recommended to replace the grommets. The invoice said they replaced the valves. The installer said he put 37 psi in them. when i checked the tires later that night, some were lower than 37 and some were higher. maybe the guy was wrong. the next day, Sunday, i put air in the tires, 41 front 39 rear. The car had been driven less than a mile, to the gas station, after being in the garage all night. When i checked them later, (cold), some were lower than what i put in, not all of them were lower. i wish i'd written it all down but didn't realize yet that i would want to remember. Each tire was different, and they weren't balanced or even, front and rear. Every time i've checked the tires cold, when i touch the valve stem on the right front tire, a buzzing noise comes from inside the wheel, for about 2 seconds. I think it also happened when i checked the right rear tire once. The noise came from the front right, and always happens when i check the front right, but i think at least once it happened when i touched the right rear--but the noise didn't come from the rear, it came from the front. The right front tire is losing the most air. The left rear also loses air but not as much. The air was much lower on some of them on Tuesday (yesterday) so last night i went out and put 41/39 in them again. I only drove a few blocks to the dark deserted gas station. when i checked them this morning, they were all uneven again. Tonight i checked them, after the car had been sitting for hours. The right front tire was 33 after the night before i had put 41 in it. The left front tire was still 41. The left rear was 37, the right rear was still 39 like the night before. I'm wondering if any one has ever heard of a buzzing or humming noise caused by touching a valve stem when the car is cold. I took it to the Toyota agency today after work and asked about the leaking air and noise. The service manager did not see a nail and suggested i have Discount Tire check it. He dismissed the noise saying these cars make noises all the time. It didn't seem like he understood that this was always at the same time, when the car is cold. Could this have anything to do with the tire pressure sensors? I guess it could be a nail in the tire, but they're brand new and this problem started the day i got them, with very little driving on them, and the other tires have lost air as well. It's not just one. The TPMS light has not shown any problem. It comes on for a second and then goes off when i start the car. I plan to go to Discount Tire on Saturday, unless i can get off work early enough to do it tomorrow. I'm worried about having a problem that no one will be able to figure out.