You state that you have bubbles in the fuel line, but is it affecting the way the engine is running? You've already touched on one cause, vibration. Make sure that the fuel tank isn't touching the frame of the plane. If so try to isolate it with foam. Also check to make sure that you don't have a leak in the fuel system. An easy way to check it to pinch off the fuel line and blow into the vent line so that there is some pressure in the tank, and then pinch that line too. If you hear air escaping, or there is no pressure when you release the line, then you have a leak somewhere in your system.

Hope this helps

Ken