I installed three rainbird valves to replace toro valves. The toros never did this. The rainbirds work fine. I have a bad problem with water hammers on all three. This occurs only when they close. I had a repairman install a new back flow unit. Its a febco, this one I believe.... http://onsmartpages.com/kelleyslawn.../productsservices/category.nhtml?catuid=10113 Any suggestions on how to cure this. Thanks for your help and patience.