I hear this sound when running alongside curbs or concrete walls with RPMs below 2000. I suspect it's a leaking exhaust manifold gasket, and intend to replace this in the near future. When I try to locate the source of the sound with the hood open and the engine idling, I cannot pinpoint any leak, possibly because of all the other noises under the hood.
Could this sound be other than a leaking exhaust manifold gasket?