Thank you, Bard and Lacine.
I think I have put insufficient information. My SMPS operates at 100Khz (97.6Khz measured). I was measuring the voltage at the output of the rectifier and filter. So its ok to expect clean DC with some ripples. But unable to read this voltage on my DMM.
Some things that I doubt are, wrong design of feedback network ( TL431, optocoupler) this enabling and disabling the UC3842. which make that audible sound.
secondly, either my transformer is radiating ,or my switching transistor is radiating so high , that my DMM is not able to take readings.
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Some more things, wont potting the flyback transformer ( with gapped core) change its inductance?