2005 astro front tires

I purchased a 05 new after about 7500mi I tookit back to the "SERVICE DEPT." and was told nothing was heard. the van now has 20000 miles the tires have been rotated about every other oil change. I get I vib. at 25 - 35 and the outer edge of the front tires are "chopped" any Iam thinking a defect in tires?

A symmetrical "chopped" wear pattern on the outside edge of a tire indicates that toe in is excessive. In the early stages of wear, an audible "hum" becomes noticeable, similar to that of a faulty wheel bearing. The "hum" will also change pitch as vehicle speed changes. As the wear progresses, the "hum" will become louder. Eventually the "hum" will change to a vibration. Unfortunately, once this pattern is worn into the tread, it's there to stay. As long as that tire is on the ground and turning, it WILL be noisy. A front wheel alignment will stop the pattern from appearing on new tires, provided all suspension and steering linkage parts are ok and the alignment is done correctly. Hope this helps.