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.
Thank you everyone for the suggestions. I am going to replace the PSU and borrow a USP with line conditoner from a friend. I will update with the results.
After a call with iFixit they sent me a new power supply. I also ordered a voltage regulator. I waited to install the new power supply until the voltage regulator arrived. So I have the voltage regulator to the wall outlet and then a UPS off the voltage regulator. The iMac is plugged into the UPS. With the new power supply in everything is running the way it should - now buzz. I have had it in for about 8 hours under off and on use. I will update after about a weeks of use on the status. Thank you all for your help!
The cleaner the AC power the better your system will run.
Check your buildings ground service and think about getting a good UPS unit.
I have been using a cheep APS USP unit and keeping the iMac on the battery. I have been thinking about upgrading that to something with line conditioning. My house was built in 1930 - wiring is a bit goofy in places. You think if I put a new PSU in and have a power conditioning UPS i might be able to fix this?
Talk to your power provider to see if they will lend you a power monitoring unit which you can plug in to an outlet to see whats happening. Try to let it run a good week or more.
Otherwise either your self or an electrician should replace the outlet your system uses and make sure it is properly wired back to your fuse/breaker panel. If the outlet is shared with anything else (other outlets) see if you can move the other devices onto a different outlet that is independent to the fuse/breaker your system is using. Also make sure its a 20 Amp service as well. Next looking at the fuse/breaker panel you should see a heavy bare copper grounding wire or cable which connects to a ground pin or the water line on the other side of the water meter (some people put a jumper cable across the meter which is OK). Make sure the pipe, wire & clamps are not corroded on the water line or the ground pin. Clean and/or replace the wire & clamps as needed so your ground path is in good shape. Try to cover the rod or pipes and electrical joints (at least were the clamps are) so they don't get wet from condensation. You may need to replace or re-locate the grounding rod if its corroded and/or not effective.
As to getting a better UPS see what the monitor shows and what you discover on your houses wiring & grounding. Often replacing a worn outlet and fixing the ground services fixes a lot of the problem. If the monitor shows you still have issues then its time to get a better UPS.