CEDAR RAPIDS — In some Cedar Rapids neighborhoods, the long-familiar thunk-thunk-thunk of a mail carrier’s truck on its way is being replaced with a far-more silent approach.After more than two decades of hauling mail and packages to American homes and businesses, the U.S. Postal Service is replacing its aging delivery fleet.Mercedes-Benz Metris vans have begun to replace the familiar boxy Grumman Long Life Vehicles in Cedar Rapids.