Missed diagnosis dilemma

I have a 95 corolla with 150K miles that has been making a hollow clunking noise when accelerating/decelerating. After having a transmission guy look at it he suggested taking it to a couple of local shops to have motor mounts and CV joints looked at. He could not find anything wrong with the tranny. I took it to the first shop and they thought the motor mounts looked a little weak (too much engine movement) but didn't act like they wanted to do the work. I guess it was too close to quitting time. So I took it to the second shop and they said the same thing. Motor mounts bad. I thought OK, then that must be it. Well $600 later the noise is still there just like before. My delimma is I have already spent $600 and the problem I wanted fixed didn't get fixed. This shop has a good reputation and I think they are honest, I just don't think they properly isolated the problem. What should I do? Forget about it or see if they will work with me money wise on getting it fixed.