All in one Macintosh

[gwb21471]

I have an all in one Macintosh computer. It use to be a performa 550 but I bought other motherboard for it. Now it is a LC 520. My question is once in a while you hear the high voltage transformer that plugs on the screen make a buzzing noise very low and then you may not hear anything for hour or two. Does that mean the high voltage transformer going bad or a capacitor going bad. I know the guy replaced few of the capacitor on the analog board. I don’t know what ones was replaced. Other than that it works great. Thanks for the help.