All of it. The 10 percent figure you occasionally see is nonsense that a lot of people believe just because they've seen it so much.
Quick Answer. Some things that cause a vehicle's front end to make a clunking noise are worn ball joints, a damaged tie rod, broken or loose sway-bar links, hard or damaged shocks, and faulty shock mounts. An automotive professional can inspect the vehicle's front end to locate the source of the clunk. Continue Reading.
Well, since it's on startup (cold) that's when the secondary air injector kicks in to help warm up the engine. There's a possibility that that's what is causing the noise. I also know there is a software update for certain cars that fixed cold start/stalling issues, it may also pertain to this as well.