Loud beep on iMac

My iMac made a very very loud single beep two times over the last week.

Also have had intermittent screen flash alerts, but don't know why.

Recently uploaded some very large folders which have used up a lot of memory and made the computer run slowly.

Deleted some of the files and it seems to have slightly improved performance.

Could it be a memory problem?

I have 287GB available of 465GB capacity