I second that you may have something loose. Check all screws, nuts and bolts, including the transformer mounting bolt(s). I once had an amp that made 2-3 rapid metallic clicks at every cold power-up. I traced the problem to an overtightened top cover screw. I had removed the top cover to re-bias the amp not long before the clicking started, which provided a clue that the noise was the result of something I had done.
