One of our clients was experiencing a popping clicking sound when Mitsubishi mini split unit was operating in heating mode. Homeowner described it as a ticking noise from new mini split indoor unit just installed. After doing much research and talking with Mitsubishi tech support we decided to apply a Teflon based oil two copper plastic area left side of coil. This appears to be the solution and we are waiting to hear back from client if a clicking noise continues.