iMac mid 2010 27" is screaming at me

I have an iMac 27" mid 2010. When it is being used to watch You Tube videos it will make a supersonic screaming sound and must be physically turned off with the power button. The computer just stops working. Could I be looking at a failing hard drive or fan? Any ideas will be appreciated.
Dennis PillowOwner G-Tech Consulting LLCAsked:
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Paul SauvéRetiredCommented:
does this occur event with the speakers off?

if the sound is coming through the speakers, then it is possible a problem the sound card

supersonic - when an object goes faster that the speed of sound...    ;-)
Dennis PillowOwner G-Tech Consulting LLCAuthor Commented:
I have seen it do this with just normal use, not using the sound. The sound does not seem to come from the speakers and when this happens there is no control for anything.

Note: Maybe I should have used the analogy of a sound like a 600 cycle generator on the flight line in the winter in Alaska....That is loud :-)
Aaron TomoskyDirector of Solutions ConsultingCommented:
Sounds like the fan. Cheap enough part unless it's the fan on the Graphics card. A store will usually charge $200 just to open an iMac and replace it. There are suction cup kits to pull the glass off yourself but be careful if you do.

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Dennis PillowOwner G-Tech Consulting LLCAuthor Commented:
Thank you for the heads up. I will try the solution.
Aaron TomoskyDirector of Solutions ConsultingCommented:
Easy test:
Get a can of compressed air, remove the memory cover  (the vent at the bottom center) then when it makes the noise blow some bursts of air at the noise, probably straight up. If the noise changes with the air, it's absolutely the fan.
Dennis PillowOwner G-Tech Consulting LLCAuthor Commented:
Will Do!
David AndersTechnician Commented:
Resetting the SMC is often suggested as a first step.
https://support.apple.com/en-us/HT201295

Google "imac fans loud"
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