Air conditioner making loud noise

Hi,
We have a Friederich 18200 BTU through the wall air conditioner on the main floor of our house. It cools the entire first floor and has been a great AC for us for over 12 years. It runs on a 240 volt line which was properly installed when we had our kitchen remodeled.
Recently it started making this loud vibrating noise like the fan couldn't get started or was somehow out of balance. I would compare it to the sound of a washing machine when the load is unbalanced. Sometimes it wouldn't even run the fan at level 1 so I would put it up to level four (the fan has 4 speeds) and then the fan would kick in but it would be really loud and vibrate a lot. The cooling part works fine, it is just the fan that is having problems.
Then it just started working again and today it is working fine again. It was broken like this for the last 5 days. All of those days were quite hot.
I remember this happening once last summer and it fixed itself and we made it through.
I would appreciate if anyone has any idea what might cause this kind of vibration in the fan and why it could just get better on its own without any service.
Thanks in advance, Steve
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snipped-for-privacy@hotmail.com wrote:

Reasonable guess is that the bearings in the fan need oiling/greasing/replacing.
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HeyBub wrote:

B4 it self destructs itself. Cross fingers the bearings are replacable.
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