The blade is hitting something. Turn off the power and spin the blade by hand and see what is doing it. Bend the offending bit out of the way.

Thanks that helps. I checked the fan with the power off and the blades spin without hitting anything. I realized that the sound is coming from the flat metal piece around the fan banging against the wood of the roof. It looks like that piece is nailed (loosely?) into the wood at the corners, and I guess it isn't secure enough. If I push the metal away from the roof slightly with my fingers, the noise stops.

Any thoughts on the best way to fix this? As a temporary low budget fix I put some of those round felt pad stickers all around the edge between the metal and the wood, and I think the noise is gone. My worry is whether water can get through that space now (but I don't think water was a problem before and that doesn't exactly seem like it was a tight seal).

Can I just run some screws from the attic through the wood and into the metal, or am I potentially causing a water issue? It's a bit tough to get on my roof unfortunately.

Great Stuff? Anyway, if I understand you there's not likely anything 'getting through'. The fan should be flashed to a much bigger extent than this "crack" on the roofing side of the sheathing.

Thanks, that makes sense. Maybe I am OK then. As a better solution, I thought I could also run some liquid nail or something around between the metal and wood. I figure this could be applied from inside the attic. But maybe I am fine with my felt pads if it holds up.

Sorry I'm a beginner at this stuff so I'm not sure what everything is called. I think my noise is actually the metal "flashing" banging against the roof. Would applying something like liquid nail between the flashing and the roof, from inside the attic, be a bad idea?