On cars with steel body panels, a different sound could indicate that body fillers have been used to repair rust spots. (articles.orlandosentinel.com)
One easy and inexpensive method to test for the presence of fillers in steel body panels is carried out using a refrigerator magnet. (bestride.com)
However, the same media outlet claims Ford is likely to stick with steel body panels for the new Explorer, unlike the aluminum treatment it applied on its F - series pickup trucks. (drivemag.com)