How do I know whether I have a damaged motor or transmission mount?

Hello again all,

I've got another dilemma I'd like to share. This one involves what I suspect to be a broken motor or transmission mount issue.

Every time I lay off the brake pedal ever so slightly at a stop and hold it at just the right position, a grinding/clunking noise from the front or bottom of the car occurs. If held at this position the grinding is constant. Not only this, but the engine seems to creep back and forth if I lift off further without accelerating. It's very similar to a manual transmission car when it jerks at low speeds in 1st gear. Albeit not as jarringly. Occasionally, I'll be parked on an incline with the e-brake depressed, shift to drive and a loud thump occurs at the same time. Sometime's it happens when shifting to reverse from a decline.

I've had the car looked at before by a mechanic who told me the 3 motor mounts are fine, but that the transmission mount is damaged. I own a Civic that would knock as I shifted which turned out to be broken mounts. No engine thumping occurs while accelerating in the Prius so I'm ruling out the motor mounts. What do you guys think based on the descriptions? If it is the transmission mount can I repair it myself?

Thanks in advance.