Hi, I recently purchased a 1999 WR400F, and today, a clicking noise started coming from the engine. It sounds like it's coming from under the valve cover. However, I just changed the oil and filter, and had to loosen up the header to do so. When I started the bike and took it out for a spin to warm it up, it started making the clicking noise after 2-3 minutes or riding, and gets louder when the rpms increase. Other than that, the bike seems to be running just fine. Would the header be the primary suspect, or am I missing something?