I have built a 3' by 4' hovercraft (with rounded corners) out of plywood with a pond liner bag skirt. it it powered by either 2 battery operated leaf blowers or 1 plug in electric leaf blower. I have encountered a problem with both power sources. The hovercraft makes a loud vibrating noise when I stand/sit on it. It sounds like when you vibrate your lips together but much louder. With nobody on the craft there is a very small noise and with somebody on it its much louder. On the bottom there is a rectangular piece of plastic in the middle (16" by 4") positioned so that is 16 inches to every side (a little longer to the corners). There are 6 approx 2" holes around the middle (2 on each long side and 1 on each short side). Any suggestions of how to get rid of the noise? I assume its from not enough holes/bad positioning of the holes but i don't know.