Need opinions from the experts again. You have always been extremely helpful to me before. The attached file shows a run at the drag strip in my son's 87 GN. He seems to get some knock in 2 parts of the run usually at lower speeds, around 50 and 70 mph. The boost is up here but there is no knock after this at higher speed and RPMs. What do you think may be causing this? He had knock issues before (blew the head gasket twice) but we seem to have that fixed. Could this be caused by the trans shifting or something else?