Top fan runs fine. When it kicks on to cool, there is a grinding sound at the lower rear of the unit. Sounds almost like a garbage disposal. Noise for 3-4 seconds, then off. After a minute or so, will try again with the same result. Also tripping the breaker...

I assume we are talking about an outdoor condenser for a/c. If so, the first thing to check would be the capacitor for the compressor. If the cap is good then you may need a hard start kit (basically a start capacitor that you add to the system). If you don't know how to check the capacitor, simply remove it, take it to a local hvac supply store and get an exact match. It is probably a dual cap (runs fan and compressor) and normally cost between ten and thirty dollars. Pretty cheap place to start. Good luck and please let me know what you find.