HI I have a 2013 ish Mt Vernon AE and the exhaust fan is squealing randomly. I took it out and cleaned it and seemed to help for a little bit but now it came back. My question is, can I lubricate it at all? It is orginal and I can see the actual little ball bearing in the back--they are exposed no cover. Could I put some graphite in there or maybe some lithium grease? Or do I just have to replace it? Thanks in advance for any help!