iMac i5 (Mid-2011) Electrical Noise Occurs when Scrolling or High CPU Usage

i'm getting weird electrical type noise refurb imac (probably why it's refurb in first place).

sounds coming directly lower right of computer. best can describe noise cricket (insect) makes; it's soft buzzing/ticking sound increases in level depending on cpu usage.

heard when scrolling or down web page lots of graphics , pictures. occurs if open large pdf document , scroll up/down. example; if open aperture , make full screen , scroll through pictures noise occur. noise occurs when cpu usage goes above 20%.

have ssd installed isn't hard drive. isn't fans because can turn fans on high/low , noise sounds same. can't power supply because located in top left , noise occurs in bottom right. don't believe it's lcd display either.

else have/had similar problem? i'm going take genius bar once i'm totally driven crazy noise.

 

just noticed today, if use mission control , select desktop sound gone when scrolling still there if cpu usage goes above 20%.

if go first desktop sound still there either way.

 

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