For the past several months, I could hear some 'puffing' sound coming out of the engine bay, but only happened when the engine was under load (in gear & driving) while warming up. Once the engine reached the normal operating temp, the 'puffing' sound dimished and went away. It's not loud, but I could hear it with windows down. Is this the 'cracked exhaust manifold' everyone's talking about? Or it's just the sound of valves opening & closing while engine oil's warming up? I have a 99 CX HB. Thanks.