Someone please HELP!!!

I recently started having a problem with my shower in the main bath upstairs. Whenever I turn the water on, no problems. When I turned the diverter to shower, the water starting coming out fine and then the pipes would make a wierd sound and then the water would pretty much shut off (a small trickle out of the shower. Happenes when shower head off too!!!). When I turn the diverter back to tub, the water comes out of the faucet fine. I can get the water to come out of the shower ONLY if I slowly and barely turn the diverter on so that there is water coming out of both the shower and the faucet. If I turn the diverter far enough to almost cut the faucet water off, I hear the noise and the water flow stops. Note: This also happens with the shower head off!!!

Now I have replaced both the hot and cold water valves as well as the diverter valve and this continues to happen. I only seem to be having the problem with my shower. It happens if I use hot or cold water.