Sounds like a typical problem with MOEN shower faucets. If you need to pull out to the handle to turn the water on and then turn it for temp control, it's a MOEN. Inside the cartridge there is a part that when shacking the cartridge in your hand you hear and fell the rattle. this somehow gets stuck. Remove the cartridge and tap it on the ground a few times and you'll hear. You can put it back in or get a replacement. There are all kind of hints on this site on how to remove the cartridge and it can be very difficult to do.
Here is an entry that I did after replacing a MOEN POSI TEMP cartridge with a DANCO brand replacement. I had help both my friend and their daughter (two homes) with the same problem as you have. http://www.terrylove.com/forums/show...NCO&highlight=