Well, I'd guess that it is probably normal. There was a stage in my PT where I'd get the popping and "grinding" sounds and they went away. If you think about it, you have to make small tears in the muscles and then they heal up in the stretched position. That's how you get your ROM back. And PT is painful so maybe that's the source of the pain. If you continue to be worried about it, make an appt. with the OS to get it checked out.