I have two Airxcel 1500 BTU heat pump/air conditioners. On one the compressor will start to rattle after running for two to three hours. At first it is barley noticeable, but as time progresses it gets louder and louder. I can even put my hand on the ceiling and feel the vibration. I have checked and made sure that everything is snugged down tight on the roof. Any ideas what could cause this problem? Next month we will be traveling to South Padre Island, Texas, and I am afraid that the unit may go out while we are there. It is hotter'n blue blazed down there in September.