First question. Was it running ok before you did the carbs?
I agree that it could be something in the cylinder. Could check that by taking plugs out and checking each cylinder with a good light and a scope, if you have one. While the plugs are out, turn the engine and see if the noise goes away. You might put your finger over the plug holes to see if you feel any obvious difference in any particular cylinder. If you do and you can't see anything in the cylinder you may need to remove that head to be sure. If you are satisfied that there is nothing in the cylinder, put plugs in and see if it runs. Just keep your finger on the kill switch in case you need to shut it down immediately.
It sounds like you may have one of the rubber boots loose and it is puffing or sucking air. Also, there are O rings where the elbow joins the head. One of them may have come off without you noticing it. Just thinking with my fingers, so to speak.