knocking in driver's side rear

What is your question?

There's an intermittent knocking noise coming from the rear of the car (driver's side). Occurs ever so often and becoming more frequent. Braking, acceleration, rough terrain do not seem to affect it...just happens under different driving conditions. Had someone tell me it may be the strut mount. Anyone have other possible ideas? I've had it on a rack and checked to see if everything was secure...no apparent problems.