I have a nv 4500 and it knocks really bad in 3rd. All the other gears are fine and I wasn't even in third when it happened. I was on the highway in fifth and I turned and shifted down to 3rd and when I hit the throttle it started making the knocking sound. I've been trying to avoid third in case something bad is wrong, but does anyone know what this could be?

 

Not sure what would be causing the knocking sound, But I would definitely pull the PTO covers off and look up in the trans to see if you can see what is loose. It could be a number off things. It could be a bearing thats bad. a syncro ring, definitely needs to inpsected before it grenands on you. Manual trans knocking means something is loose.
anaheim gear is a good place for parts for these transmissions. anaheimgear.com

 

Well I pulled it apart last weekend and it turned out to be teeth missing on the counter shaft.