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

sounds like something is puffing? Gasket? Spray the sealing part with WATER and see if anything changes or bubbles up. Add a little dishsoap in there so it will bubble over an air leak. remember under the carb too, its a huge source of exhaust leaks. If it has an aluminum intake, check for cracking of the heat stove under the carb, itll give you a massive vacuum leak. And of course check the valve lash.