So after we returned from our last camping trip, I've noticed the tires on my van are making more noise than they used to. Like a slightly louder, slightly more high-pitched sound than before. Gets higher pitched as I drive faster and lower pitched as I slow down. I checked tire pressure and it's fine. We were towing with the van level, thanks to our new airbags installed in the springs. Any ideas why tires would sound weird now? Did they somehow get worn down unevenly from towing?