I had my 4th ACLR about 10 weeks ago. I used my hamstrings for my first recon on my left leg about 5 years ago. Unfortunately since then I have had 3 more recons. I never had any hamstring problems but it sounds like you may have ruptured or severely stretched yours. You shouldn't do any hamstring exercises for about 6 weeks after the surgery because they are weak and vulnerable to tearing and stretching. The other possibility is that your surgeon did not harvest the hamstrings properly which has lead to subsequent injury. Although hamstring harvest is done frequently for ACL reconstruction, many docs do not do it properly. Often times the docs cut through the wrong tendon. If I told my surgeon that I heard a popping sound like that and had extreme pain he would have me in for an Wikipedia reference-linkMRI right away.

Good luck