it started after the computer had been off for a few hours and was cool. As soon as I pushed the power button, the noise started. It sounds like a jet taking off. It didn't stop so I powered down the computer and am hesitant to do anything. It is a 6 year old Intel iMac and out of warrant. I did read about how one could do a hardware test by holding down the D on startup but that didn't work, just got a gray screen.
First reset the SMC as per > Resetting the System Management Controller (SMC) and then try starting up.
If it does the same thing, run the Hardware Test as per > Using Apple Hardware Test - Apple Support
First, perform a SMC reset to make sure it's working fine.
The sound you've described may be a fan noise - an inexpensive repair. Or it could be a hard drive, since their sound can vary quite a bit.
In any case, I urge you to back up your sensitive data (if you can). I'd proceed to a reliable repair shop or Apple Store.
After posting, did some more digging and found how to properly reset the SMC which required disconnecting the keyboard now if it would just stay on and not keep shutting off. At least the fan noise is gone.
On startup iMac makes loud whirring noise, how do I fix it
