If you were any closer I would loan you my Dranitz but that is the answer Rent or borrow a logger and track it yourself. Once you get a pattern, you may be able to track it down. If you are just willing to user a meter, an analog one is better than a digital, even a pretty cheap analog. You are just looking for the change, not an absolute value. If you watch it you might develop a pattern. That is what a logger can do.
If you are just looking in real time, hook your clamp to the HVAC compressor circuit and watch the voltage on the bus with your cheap analog.
