engine knock

Can anyone give me there suggestion what this Knocking is? I checked out the roller rocker and none seem lose and all the rods looked okay? Possible something to do with the automatic transmission plate? The Noise as seemed to got worse after I took the rocker covers off. I posted three...

I have an 03 suburban and it will knock on start up if I don't fill it with highest octane gas from Shell, Chevron, Or 76 Stations. The knock will last for a few days on start up, then go away. Does anyone have the same problem? Besides filling only at these stations, does anyone know if there...