I just found a hard lump on my left knee, on the outside left corner of the bone under the knee cap. no pain but it's protruding like like a marble.

Don't know u age.: painless lumps around joints - could be harmless conditions like bursitis(an island of cushion like structure lined inside by the same lining as the joint called synovium) , joint cyst/bone cysts , joint effusions appearing prominent and tense almost like a solid selling .But also need to r/o bone related tumors as a part of completion in working up the diagnosis.Get an orthopedic consut . opinion

Needs to be checked: It needs to be checked . We need a detailed history if there is any trauma .is it painful to touch and any other associated symptoms. It needs to be examined and may need. X-rays or other imaging .It may be a hematoma , a cyst , or a bony tumor .So get it checked by your Promary care physician u