Originally Posted by rcassidy
there is a "buzzing" noise
I'd use a garden hose to isolate the noise source. You need someone on the distant end to signal you when the hose end is on top of the noise.
Assuming it is not a 60 Hz hum, it sounds like a relay armature oscillating, possibly from a high resistance connection upstream of the compressor.
The connection holds until heavy current is drawn, the voltage drops, the relay drops out, the voltage rises again to an approximately normal level, etc. The heating of the connection due to the heavy current draw causes it to "heal."
You get the same result, a rapid clicking noise rather than a buzz, when you try to crank a car engine with high resistance battery connections.
Knowing the length of wire and gauge supplying the compressor, you can figure out how much voltage drop you can reasonably expect with a known load at the compressor end of the cable that supplies it.
The problem is finding a heavy, 240v known load; most toasters/hair dryers want to see 120v. Two identical hair dryers in series might work to give you a known 10A load.
A cooktop element also works but you'd have to find a way to hold the thing for the few seconds it takes to measure the voltage drop.
Connect and disconnect the load a few times. You're looking for the difference in voltage.