So to get started ive had a 2003 Forester XS SOHC for the past few months. Its been a GREAT car. Ive posted about it before but the whole post dissapeared after this new update to the site. So here I am again but with some new news. Oil pressure is good, at idle and warm it is between 12-15 psi. 65 when cold. Checked the oil after a few hundred miles, no metal shavings. The upstream O2 sensor is bad, I know that. But that is the ONLY code its throwing. I replaced the fuel filter. But on a cold start, it is quiet. No sounds other than the intake and fuel injectors. Until I give it just a little gas and you hear it tap between 2k and 3k. When it warms up, its constant. Idle is about 650. Makes the sound up to about 2k then you cant hear it. No loss in power, gas mileage is around 24mpg. If i drive for about 30 minutes and stop, the knocking is gone. I let it run for a few minutes or shut it off and turn it back on, it comes back. Im clueless. I checked the tensioner on the timing belt as well. Still nice and tight and doesnt move a bit. If anybody has any help or advice, id immensely appriciate it. Thank you.