Why does my left elbow making popping sounds when I lift weights?

Elbow popping. Can be due to some tendons rubbing over eachother or a bony prominence while under tension, or due to some distraction inside of the joint during lifting that allows gas to escape from the joint fluid (similar to cracking our knuckles). Sometimes, the joint is not sliding- but 'jumping' thought a range of motion under heavy load, but that could be damaging the joint surface. Make sure you stretch!
Tendon. Usually from tendons that snap over the bone. Sometimes with ulnar nerve will snap over the bone on the inside of the elbow. As long as you have no pain or numbness there is nother to worry about.