I would download and install the excellent and Free App below to check your hardware component's temperatures inside your Mac, and you can post those temps here for comments:
Yes, the fan was making noise.
Fyi, 10 months ago my iMac won't start-up. I sent it to my Apple reseller to have it fixed thinking that it was a power supply problem. It turned out to be the logic board problem. Apple replaced the logic board for me for free. At that time I had asked them to fix the fan noise as well but after changing the logic board there was no more noise from the fan and I no longer had the problem of not being able to turn on my iMac for several hours after switching it off. Then 3 months later the fan started making the noise again and now occasionally I won't be able to turn my iMac on after switching it off.
A speed of 1000 RPM should barely be audible but the values reported may not be accurate, if their tachometers are inop.
Run Apple Hardware Test:
Report any errors it finds.
You must remove the firmware password, if one has been set, before you can use AHT.
The inability to start your Mac may or may not be related to this, but any problems related to unexpected system sleep, shutdown, lights or fans justify an SMC reset:
Before Resetting the SMC
Try each of the following steps in this order before you reset the SMC. Test the issue after completing each troubleshooting step to determine if the issue still occurs.
- Press Command + Option + Escape to force quit any application that is not responding.
- Put your Mac to sleep by choosing the Apple () menu from the upper-left menu bar and then choosing Sleep. Wake the computer after it has gone to sleep.
- Restart your Mac by choosing the Apple () menu from the upper-left menu bar and then choosing Restart.
- Shut down your Mac by choosing the Apple () menu from the upper-left menu bar and then choosing Shut Down.
Resetting the SMC for Mac Pro, Intel-based iMac, Intel-based Mac mini, or Intel-based Xserve
- Shut down the computer.
- Unplug the computer's power cord.
- Wait fifteen seconds.
- Attach the computer's power cord.
- Wait five seconds, then press the power button to turn on the computer.