The Detroit News reports the National Highway Traffic Safety Administration is investigating almost 400,000 Hyundai Sonata models over rust complaints. So far, NHTSA has received six complaints that Sonata models, built between 2006 and 2008, have allegedly suffered rear control arm failures due to subframe rust. Three of those occurred at highway speeds, and two resulted in the actuation of the vehicle's stability control system, though no one was injured. An investigation is typically the first step toward determining whether or not a safety recall is necessary.This isn't the first recall for the Sonata. Last year saw Hyundai recall certain 2012 and 2013 models for side-curtain air bags that could deploy inadvertently, and the automaker issued a campaign to recall 139,500 Sonata sedans for steering issues back in 2010.