to be in limp mode there should be an engine code. Oil pressure or knock sensor could possibly sent a code but other than that, a rod knock is mechanical. If you're in limp mode scan for codes and see what the computer is looking at.
rod bearing, cracked or broken flex plate... things break. you should have a shop listen to the noise and see if they can verify what the noise is coming from. Your other post said something about oil pressure. Is the oil pressure normal? Low?