If the buzzing is coming from the area around the top of the bulkhead panel by the rear window, before you spend the time to remove the bulkhead panel you might want to try something different. While standing outside of the car, take a business card and insert one end of it in the gap between the forward edge of the rear clam and the bottom of the rear window glass. Try sliding the card from side to side along the entire length of the glass and if the card gets stuck or pinched anywhere along the way, you have found the problem. I had a nasty buzzing at about 3,500 rpm that sounded like it was coming from the bulkhead panel and it really was the clam touching the rear window and vibrating against the glass at that rpm. For the longest time I was convinced the rattle was coming from inside the car behind the bulkhead panel, but once I stuffed some rubber strips between the glass and the clam at the spots where they were touching, the buzzing completely disappeared. Lotus had simply failed to trim enough off the forward edge of the window aperture on my rear clam. It will only take 30 seconds to do the test with the business card and if you find the problem there, you will save yourself the hassle of removing the bulkhead panel. Good Luck!