Q: Normal startup?

Hello,

If your iMac has a built-in optical drive, it might be what you're hearing: Mac notebooks with optical drives: Noises from the optical drive - Apple Support

Please listen to them sample files.

Thanks for your response,

From the way you describe the sounds it appears to me like it could be static discharge, though this is not common for iMacs, especially not during every boot. The flashing of the screen also should not happen, unless you've got an external display connected or installed software that forcibly changes the appearance of the display.

Do you happen to have a video of the problem? Maybe upload it to youtube? If possible with the sounds as audible as possible.

You have a spinning metal disk, that makes a noise at irregular intervals, get used to it.

During the start phase there is a moment when the boot switches over to loading user preferences, it comes second in the order, a blip at that point is quite normal.

Just enjoy it.

"...in the middle of the progress bar..." 

That is where the firmware is read and started and where the OS is going to be read (from the disk) and started. This point looks like the system is hesitating there.

I never "heard" anything anything there, but I have banned Fusion Drive a long time ago and have SSD large enough to have everything on it. Your mac has a HDD and a small SSD with a special OS on it. So it is probably as Csound1 says "get used to it".

Just don't switch it off, most problems go away if you do that.