We went to the lake this weekend and I tried hitting the biggest wakes I could to duplicate the noise pet575 and his brother had. One time my bimini top hit the tower and made a noise similar to what you described, but not as loud as hitting it with a hammer would make. Does yours do it everytime you hit a big wake? I also tried taking some more pictures but didn't do a great job. I did get in a picture of my 4 person tube that is a blast. I'm really loving my Four Winns and wish I could go out every day. I boat in Greensboro, NC at Belews Creek Lake.
http://s231.photobucket.com/albums/ee91/calvinhead/