What you describe is what mine was doing earlier this year. Only knocks when holding steady speed on the road. I dropped the oil pan (before I drove it too long like that), and found that I had spun the #4 rod bearing. The other three were still good for having 250k+ on the engine. I checked the journals(for roundness and overall wear) and replaced the bearings, and it's still running (knock is gone). I am still going to swap the mains out, too, but it's still going for me.
