There would only be two occasions where you would hear the fan pull at 100% (i.e. jet engine); when the engine is very hot and close to overheating or the clutch has failed and is locked up permanently. Did you check the fan fuse? Don't just visually check it, use a multimeter or test light. It could also be the relay. You can switch it with one from another circuit that uses the same type (don't remember which one, maybe another member can chime in). You could also temporarily put 12V+ to the white wire at the fan while the engine is running and you will hear the fan fully engage and then release after removing the 12V+.
These fans were problematic in the first years and yours would have been of the worst ones if it's still original.