Chosen Solution
Ok so about 6 months ago I replaced the power supply in my iMac with one purchased from here. The guide was great, took no time to replace and while I was at it I added a second drive (256gb Solid state with the cable purchased from iFixit). I was supper happy for a few days untill the dreaded buzz noise came back. Last week I went ahead and replaced the display LED driver board hoping that the buzz would subside. It did not. Noise only seems to happen once it warms up. Once the buzz starts it does not stop untill I shut the mac down and do not come back till the next day. I wanted to know if anyone had any advice before I purchase another power supply or a new iMac / pc.
With you using a cheep APS USP unit and keeping the iMac on the battery, you have protected it. You say you purchased the power supply from iFixit, this one here. This comes with a 1 year warranty. Follow the return procedures given to you by iFixit, they will replace it. There is no need to buy another one.
The cleaner the AC power the better your system will run. Check your buildings ground service and think about getting a good UPS unit.
