One of two things are the most likely cause.
Tank is mounted to firm, vibration transfers to the fuel and makes bubbles. Mount the tank loosly in foam.

There is an air leak somewhere between the clunk and the carb. loose fiting fuel line from tank to carb or a small hole it. Same for the tube inside the tank, the smallest hole that you cant see will do it. If you have to, and if you can, put the whole tank and lines in a bowl of water, plug one line and pressurize the other line to spot the leak. Could also be a hole in the tank. If your sure the tank is ok, replace "all" the tubing.