Help!! Firebird engine rattling

I have a 96 v6 firebird and it the engine was knocking for about a week and i couldnt get it to a mechanic. Now when i start the engine the rattles really bad in gear and especially when moving. I have recently parked it and am going to take it to a mechanic but i am wondering what could be wrong. I think that it may be a broken valve spring. Please tell me what you think it could be