My wife noticed a metal rattling sound coming from the muffler when standing outside the car when I was pulling into our garage - I checked it out and it turns out that a metal rattling/slight scraping sound lasting about 2-3 seconds could be heard every every 10 seconds or so when our 2003 3L H6 VDC Subaru Outback is in drive. What could be the cause? It seems to only be coming from the muffler - can't trace the sound to anywhere else under the car.