If it helps,.. I have the same knock. I've heard it since I had about 110k, on the odo., at about 140k , I got fed up with it and had my engine diagnosed. It turns out the engine was performing above average for a car with that mileage. So I Asked my mechanic if he had an idea where the sound was coming from. He said it was coming from the main bearing, and that If I keep up with all tune ups and such, the motor would last a life time. He also said, that he saw it in almost every Accord he dealt with. I went for a second opinion, and the other mech. said it was a rod or bearing. Both mechs said it would be hard to diagnose, and the cost to diagnose, would match the cost of the replacement of the part/ labor.
I know have 165k on my odo., and the knock has not gotten any louder.
Although, I do use Mobil 1, and Sunoco 94 all the time.