Changed both fuel filters yesterday and discovered today the engine seems to have more of a rumble sound. Checked the exhaust system for leaks and broken hangers. Checked the air filter and housing. I know these engines have subtle sound changes after service but this is a first. No idle problems, power loss or smoke but it seems a little rough around 2500 rpm and higher. The rumble seems to happen on hard acceleration. It's not loud like a leaking muffler. However that sound describes it best. Air in the fuel system? Poor non-winterized diesel? The car is due for a valve adjustment. 1984 300D