I agree with
@Henro that your trouble is in the mower or mower drive system.
There are three (or more?) spindles that hold the blades. They are driven by the deck belt. You should be like that one goat and give a good look between the tractor bottom and the top of the mower deck to see if something got jammed in there and is causing the noise. If there is, then get it removed from that area. Also, look for the pulleys all being straight and the belt running through them properly. Look for places that paint is scraped off around the pulleys on the deck that would indicate rubbing.
If nothing on top then look underneath the deck (through the chute?) and see what you can.
If you don’t see anything on top or bottom, then I would assume that one of the spindles is failed. There are a couple of bearings in the spindles that need to be greased often. The center (driven) spindle is probably oil filled and will have some gears involved. You would probably want to remove the deck and inspect the spindles but you may be able to get the deck belt loose and then turn the blades to see if it is smooth turning or makes noise.