Well, there's plenty of rural and 'rich suburban' areas close to freeways. In my county in Florida, they're building brand new 500k dollar houses 100 feet from the freeway. For reference, 500k is high end of house prices here.

Would be interested to see if those show the same.

If not, you could rule out white noise completely. That would then seem to point to other factors, including total pollution, or more socioeconomic things.