Cannot determine cause of loose buzzing noise?

I can offer two suggestions. The first, based on the speed where this occurs, is to temporarily tie up the radiator fan. If the noise no longer occurs, replace the fan motor. Worn bearings will do this when the wind spins the fan blade. I spent hours the first time I ran into this.

Second, look for a tool called the "Chassis Ear". There's a few different models now. The original one consisted of six wired microphones that you clip to suspect parts, a switch box to select one microphone at a time, and a set of head phones. By switching between them, and moving them around, you can zero in on the source of the noise. Be aware many mechanics have never seen or even heard of this tool. The newer models have some wireless microphones.

First check at some auto parts stores that borrow tools to see if they have one. You will find them on the tool trucks that show up at every repair shop once a week. I found mine at a better price on eBay. It's a very effective way to find the elusive causes of squeaks, rattles, and other noises.