Sometimes my left shoulder grinds when I lift weights, even with light weights.

While grinding isn't normal, it is common. It can signal several different problems, including a problem with the acromio-clavicular (A-C) joint, shoulder impingement, or a rotator cuff tear. If the grinding is on the top of the shoulder, one to two inches in from the side of your arm, it's probably a problem with the A-C joint—bone and cartilage are rubbing together. Another sign that this is the culprit: You'll feel discomfort in your shoulder if you do a bench press and bring the bar all the way down to the chest, forcing your elbows behind the line of your body. For grind-free lifting, try modifying exercises by limiting your range of motion: During a bench press, don't bring the bar all the way down to your chest; during the side raise, don't bring your arm all the way up to shoulder height. While you can continue working out, avoiding movements that cause discomfort, it would be a good idea to get your shoulder checked by an orthopedist before the grinding turns into something more serious. Many shoulder problems can be helped by physical therapy, while some (like rotator cuff tears) may require surgery.