AC compressor makes strange constant noise, won't run
Given that you are likely talking about a residential air conditioning compressor with a single phase motor, that sound would be indicative of a failed Starting Capacitor in the compressor motor, so the motor is not able to start. Many of those are hermetically sealed compressors, so the capacitor is separately mounted somewhere away from the motor itself, then there is a "Potential Relay" that takes it off-line once the motor has started (as opposed to a centrifugal switch found on standard single phase motors). So if the capacitor is OK, it might also be a failed potential relay. Either way it's not something the average homeowner can fix, it takes some specialized knowledge of what to look for and how to troubleshoot it. I suggest calling an HVAC service tech.