I've had an LG 12 BTU unit installed at home for about two years now. Recently I noticed that the indoor unit started making a strange humming sound, especially when I turn it up to the max. It vibrates right there on the wall. Could it be that some fastener came loose, or is this normal for a unit this old?
I'd second checking the bracket, but also try looking at whether the fan in the indoor unit has gotten clogged up - accumulated dust can cause vibration and change the operating noise.
First check the bracket of the indoor unit - usually vibration and humming come from loose bolts. Grab a wrench and tighten all the fasteners, they tend to come loose over time.
If that doesn't help, the problem's likely inside: the fan might be worn out or the compressor piston could be hitting. That's not something you can fix yourself - you need a service technician to diagnose it, otherwise you risk completely breaking the unit.
Is this a recent thing or has it been humming all along, just more noticeable now? I've had an air conditioner running in my house for ages, and when it started sounding weird, the first thing I did was pull off the front panel - there was a ton of dust buildup inside from over the years, and the filter was clogged. After cleaning it, the noise almost completely went away, and the vibration too. Of course, tightening the fasteners is important as well, but people often forget about the simple stuff like a dirty fan and evaporator.
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