2000 Mustang GT rattle under throttle.....
 

So ever since I bought my car it has had this rattle and I’m trying to figure out what the source is before I put many miles on it. Good news is it’s by no means a grinding noise so might not be a severe issue. Here’s the deal though, it only rattles when you apply throttle and it isn’t under load. Like when I accelerate hard and the engine is under load, no rattle. But when I barely crack open the throttle just to maintain a constant speed, the rattle is constant. Im guessing it’s a valve train issue but I’m not sure. So far I haven’t been able to locate any loose shields or anything. Any suspicions as to what this might be and why it only rattles at certain times?