House is cooled by two separate units, both installed 10 years ago.
Recently, the larger of the two outside units will make intermittent grinding/banging/clunking sounds. When this happens the lights will flicker for the same period (could be 5 - 30 seconds). The unit still provides cooling, but this noise is annoying and disconcerting.

A service tech has come on two separate occasions, but as luck will have it, it does not happen when the tech is here.

Can anyone provide any help in troubleshooting this problem and/or give me some advice on how to properly explain this to the service tech next time I call?

Thanks,
CJ

This is a type of problem that you have to be there and hear it to tell for sure, the compressor might make this type of noise if it is hard starting or the fan if it has a bad capacitor causing it to start hard, or motor bearings, look at the fan blade to insure that it has not come loose. Check the fan blade and listen to it to determine where the noise is coming from. Good luck , Mike