iMac 27" Late 2009 freezes during start up

This started happening a few months ago and became progressively more frequent. It got the point where it now only boots into safe mode.

So I ran Apple Hardware Test and it would fail and give error 4MOT/4/40000003: HDD-1342 (the last four digit number would vary when the test was run again).

I Googled that code and found some different approaches to trying to solve it. One person said the HD fan was loaded with dust and cleaning that could fix it. So I opened up the iMac and blew out two cans of compressed air—yes, there was a lot of dust in there. Now Hardware Test now passes without error, yet the iMac will still freeze during start up as it had before.

So I tried swapping in a different HDD with a fresh install of Mac OS 10.11.4 (most current as of now). But the same issue happened. It would freeze with the boot progress bar a little past halfway, yet you could hear the male assistant voice speaking as it should upon first boot of a fresh OS install.

I then tried an SSD and same issue.

Some think it's the HD fan, but it works fine, and even better now that it's clean I assume.

So maybe that "4MOT..." error code was an unrelated issue to what's causing the freeze? Maybe that's a fair assumption to make since it's passing the Hardware test now. Any ideas on what the freeze is all about?