When I turn it on it makes a noise as soon as compressor starts. It's a humming repetitive noise. I thought it was the fan, but when I put it in fan only mode it does not make the noise. Now when the compressor is on, it makes normal noise but lately it started to get a little louder with the repetitive humming. I want to attempt to reduce it by opeining it up and tighteing the screws, oiling what I can ...etc. My question, is what else can I do?