TL80 engine knocking

[halfbean]

I have a 2001 TL80 with the naturally aspirated 238/3.9 engine and 800 hrs. Was cutting hay today at about 2200 rpms and it started knocking. It sounds like it is coming from the back of the engine. It did not get hot and the oil looks fine, there are no leaks. The knock is pretty loud but the tractor will start and run. I started it and moved it away from the haybine and shut it down.

Any idea what it could be and if it needs a rebuild how much should we expect to spend?

Thanks for help