Throbbing Knee

Patient: Why is my knee throbbing and what is it a symptom of?

Symptoms: Upper right side of left knee throbs/twitches visibly and at different pressures and speeds.
Feels weak when walking after or during tremors.
Throbbing occurs while I am sitting or while my knee is bent.
Throbbing is continuous for a few minutes, ceases or slows, and then occurs again.
Throbs often throughout day.
NO discoloration, NO temperature difference, NO previous injury, NO sweeling.
I had an arthroscopic reconstruction on my other knee last year, so my left knee may be overused.

Doctor: