Norcold N611L Clicking

A while back my Norcold N611L stopped cooling. Would cool sometimes but not all the time. Also heard a clicking noise at the circuit board. Changed the circuit board. Still the same. Had it turned off for a few months and turned bak on a few weeks ago. It's cooling fine now but it clicks a lot. Here's what I've noticed but haven't found the solution. I have two 6 volt batteries hooked up in a series. When I turn on a 12volt light the clicking stops. Another thing I've noticed is when plugged in to shorepower my furnace blower runs slow until I turn on a light and then it speeds up. The furnace blower seems to work fine when not connected to shorepower. Why would turning on a light boost 12 volt power to another appliance? Converter bad? Bad battery? Thanks