I recently took my Infiniti EX35 to mechanic, the past couple weeks the car seems to have a noise when driving. Mostly can tell at higher speeds and when I take foot off gas and let coast. I thought it was exhaust/muffler but mechanic stated no leak was found. He thought possibly it could be shields/baffles within muffler/exhaust but not 100%. Is there any other potential cause? It is only a 2008.