When i start my car and its cold(first time of the day) occasionally ill hear a tapping/knocking sound when i give it gas. Under a load it is louder and changes with RPM. but by the time i get to the neighbors mailbox its usually gone. And i do have good oil pressure. Recently ive been giving the car about 2 minutes warmup to avoid the sound. But im worried about what it could be. The engine has 186,000 miles and has Pennzoil Syn 5w-30 in it. I change the oil every 4,000 miles and this has just started. But its only once every few days it does it. I thought about trying motor flush when i change the oil next but im not too sure about putting stuff in my oil. Has anyone else had this problem or have any possible solutions.