help with a vibration noise

My engine is generally running and sounding healthy. However, around 40-50 mph I hear a rattling sound that sounds like it is under my feet. I think I have determined it is coming from the rod (and assembly) I have circled below. I'm embarrassed to admit that I'm not positive I know the name of this part. I think it is the transmission throttle linkage, I'm sure someone will educate me.

Engine is '72 225 with 36K miles.

Any suggestions about how to quite things down?