If your tires which are now on the front spent considerable time on the rear of the car, they may have cupped slightly. This occurs from oscillation (bounce) going down the highway and will create a high/low pattern on the tires. The noise becomes apparent when the affected tires are moved to the front, where the extra weight and closer proximity to the driver makes it very obvious. On many small, lightweight cars like the Civic, you have to be quite vigilant with the tire rotations to keep this from happening, as in, do them more frequently or physically watch and feel for cupping and be sure to rotate the tires before it becomes bad enough to make noise and irritate you. New struts and an alignment may also help prevent this from happening again, but I won’t guarantee it. It’s not dangerous, just annoying, and they should eventually wear flat as long as they are not severely cupped.