hi, don't know if you got this sorted, i just happened to be perusing the monitor forum and saw this.
the NT500 has an SMPS power supply that has an overcurrent protection built into the main switching FET. if the output goes too high it will shut down (hence the tweet or chirp)
if the fault was with the SMPS part of the circuit, it would likely not make any noise at all.
in this case, tweeting/clicking,chirping... is all good, it means the SMPS IS working as it should.
the best way to start troubleshooting is to hit up the output diodes on the low voltage side of the SMPS. lift out the cathode (banded) side away from the SMPS transformer (T501)
D508 runs the 140v B+ system (HV)
D509 runs the 51 volt system
D510 (C551) runs the 15/12 volt systems
D502 runs the 5 volt logic systems
likely your issue is going to be with the 15/12 volt system... if lifting out D510 (and remove or lift out one side of C551) allows the SMPS to get up and run (just run it a second don't run it long at all or you may damage other systems. basically just long enough to verify the SMPS is not ticking) your issue is in that system (likely IC301, R623, R623A, C323 etc.) if it's another system go from there.
you can lift out each individual voltage out of the circuit and find out which one causes the SMPS to stop.