Surgery done 2 years ago, need not be worried about anything of that. This sound can be just a tendon slip during pushups, over any bony prominence. That means, when u go in one direction, the tendon moves, when u come back tendon also comes back to original position and this makes a small tik sound. It is painful because of constant rubbing and wear & tear in a minimal intensity.
Next Steps
Apply ice packs over the area after workout, for 10-15 min, not more. If possible, avoid this particular exercise, and give rest for that tendon. apply ice twice a day to enhance healing. once u feel u ready, u can start the exercise again.
Health Tips
Consult a physiotherapist in person if pain is intolerable, can get treatment with some electrical modalities and manual therapy.