Intermittent really loud noise problem with iMac 27 inch

Hi all,

I have an iMac (27-inch, Mid 2011) with a 2.7 GHz Intel Core i5 CPU, 16 GB of memory. It's running macOS High Sierra. I recently upgraded it to an OWC 1 TB SSD drive, and rebuilt it using a Time Machine backup from from the original HDD.

It's now occasionally locking up and creating an incredibly loud, repeating sound (digital, not mechanical) from the speakers, and requires a hard reset in order to recover. It can be days or weeks between this lockup event, and so far I haven't noticed a pattern about what I might be doing at the time when this happens. It has happened once when the iMac was in sleep mode. In between crashes the system seems to run normally.

Has anyone else experienced this, and probably more importantly, has anyone got any pointers, suggestions or ideas about how to solve this problem?

Thanks in advance!!

Doug