Hitachi EC12 Compressor- weird hum

Make sure the power cord is plugged into an adequate extension cord if one must be used. Never less than 12 gauge wire.

The start/run capacitor may need to be replaced.. If there are any ball check type valves, make sure they are not worn or sticking. they would be on the output of the compressor head itself.

I have the same compressor with the same problem. I have temporarily eliminated the thermal reset with a piece of 12 gauge hard wire and it runs just fine. However, I do want to replace it before the warm weather come to the north. I tested the draw at 15.5 amps and the reset was a 17 amp. I am told by someone with superior electrical background that I need to install a 20 amp thermal reset in it's place. I have found that mine runs best when plugged directly into a 20 amp circuit and when using an extention cord I have a 10 guage cord that will carry the load needed to run the EC12. I have also changed to 100% synthetic oil.

you may have some water build up pull the cover apart look at yor pistons for build up

how do you know one tank drops to zero unless you have a pressure gauge on both tanks sounds unlikely unless you are looking at the gauge on the pressure regulator

well make sure air pressure is not too high. if not just replace it any air supply house carries them

It looks like there might be a knob which sets the max air pressure coming from the compressor. If there is one, this may have been turned down.

There are usually 3 pressure settings on an electric compressor. Compressor max, pressure switch cut on, and pressure switch cut off.

most likely the the reed valves are stuck or miss aligned. Worse case senerio your piston seal blew causing this not to build pressure. Read manual on how to disassemble head. to remove should be 4 bolts and your copper line, just be careful of the reed valve plates, they are thin steel that sit into the head. Put back the same way you took it apart.

There should be a pressure switch adjustment under the cover of the switch remove the cover and reset.

Cheap oil-less compressor, normal.
If you don't like it, buy one that uses oil and a drive belt.

It sounds like the air pressure assembly unit is not reading the pressure drop. some of these units have adjustments under the covers and be reset.