Villa:

I now see the probable cause of the rear prop's noise. You have made an inefficient siren.If you can cut the trailing edge of the center section, put an extension on the rear engine's propellor shaft, anything to increase the gap between the t/e nd the prop blades should help to quiet it down. Even moving the engine rearward on its mount. Or get one of the long mounts to get it further back.

Zinger propellor:

http://www.zingerpropeller.com/Pusher.htm

lists 9x4 through 9x7 pusher props for $2.90 each, pack pf six. Their range of two blade pushers runs from a 7x4 to a 24x16. They also have just about any size three blade pusher you might want.

Hope this helps.

Bill.