• "Pop" - Many patients, but not all, will hear or feel a "pop" when the ACL tears.
  
• Immediate onset of swelling - This is an indication that there is bleeding from the injured ligament.
  
• Pain - Most patients experience quite a bit of pain with an ACL injury.
  
• Instability - Patients often describe a buckling or unstable sensation in the knee.

see "What is an ACL injury?"