'71 198 swap to a '76 225 - help identifying noise?

The original 198 in my '71 Valiant lost cylinder #6, so we found a decent '76 225 and swapped it in today. Mounts were different and there was a bunch of additional emissions stuff which we removed, but otherwise went pretty darn well! These engines are so straightforward, love it!

Anyways... it is making a slight noise / tick. Can any highly trained ear out there point me in the right direction? Don't think it's simple exhaust leak.