slight knock at start up
 

I hav 04 tahoe. It has slight knock on cold engine start up, #7 plug is fouled with some oil and soot and electrode tip has a white lean mix look to it. #7 cylinder only, all others r good. Compression is 120 ish on all cylinders. It does use 1qt ever 1000 ish miles. Doesn't smoke on start up but once its warm if I accel hard it does give off black smoke around 5000 rpm. Could it be leaky valve seals, weak /plugged lifter or bad injector that is giving rich mix on hard accels? Oil press. Is in normal range and no check engine light. Ideas anyone?