2007 Ford Fusion My car seems to sway/swerve

2007 Ford Fusion 4 cyl Automatic 49900 miles

I bought my 07 fusion about a year ago, and I've noticed that when I'm driving down the highway (usually any speed between 40 and 80) my car seems to swerve or sway left and right. At first I thought it was just wind blowing it back and forth. Then I realized it does it regardless of wind, so I had the tires aligned, and that has not helped. I don't want to have to spend lots of money doing trial and error, so I hope this sounds like a specific problem!
Thanks for your help!