The noises are volume dependent - i.e. the volume of the noise changes with the volume setting  They start when the machine has been on for 15 ~ 20 minutes  They are not continuous - they start and stop randomly, with some 20~30 minute gap. There is no other buzz or hum or any background noise.  The noises occur whether the machine is in Aux or CD mode and even when nothing is playing (Radio mode not tested since the radio will mask the sounds)  Tested with machine in Aux mode - with nothing connected and CD mode - with no CD and sounds still occur  I opened up the machine and ensured all connectors are tightly fixed, so no issues there
Any help in zeroing in on the reason for the noises would be appreciated.