Lb7 loud noise coming from oil fill once cap is removed
 

Ok so I took my oil fill cap off and there is a terrible sound coming from within the engine. Sounds like a bearing but the truck runs fine. I've taken it to the dealership and another mechanic and both have never heard this sound before! If any of you have heard this sound with the oil fill cap off what was it? I have video of it but don't know how to upload. Please help!!