I had a similar rattle that was driving me crazy. I was convinced it was in the tank. It turns out it was the ears on the radiator where the support rods connect. The rods would then shake and transfer that vibration to the firewall near the gas tank and that is where I heard the buzz.
Thanks for the good ideas. The vibration occurs when the engine is revved to around 300-900 RPM, even sitting in the shop. I checked the speedo position, it is not the culprit. But, I will check the things you all said to try, maybe I'll find it. It seems to echo on the left side of the firewall when I'm inside of the car.
Thanks again.