Many of the Classic/SE macs I saw in for service years ago would develop cold freeze cracks in the solder of the power supply connector on the analog board. This is the vertical board on the top half that has the connector for the CRT. Look for 4 or 5 large diameter solder joints in a vertical row near the top edge. When they failed, the analog board would make a high frequency hiss/whistle noise. Re-flowing the solder on these joints was a cheap and reliable fix for many machines that crossed my bench back in the 90's.