I did a cleaning of the fuel system last weekend with a can of diesel purge. I took a trip the other day up a steep incline on the interstate. It is I-77 going into Virginia and has about 4-5 miles of high grade up the mountain. Half way up there is a noise that sounds like an exhaust leak. A visual inspection of the exhaust finds nothing. The sounds only occurs after the engine has warmed completely and been running for a half hour or so and then only when the engine is under full load in 4th gear (my car is a manual transmission) especially at highway speeds. Although I could still hear it when pulling into a gas station in another gear. The exhaust manifold gasket was replacement about 4 months ago. I am at a loss as to where to start to look for possible causes. This noise is engine speed related as when I let off the gas it goes away.