Hi, I don't think we say either of those... I think "at the stroke of noon" would be the correct one however it is not a commonly used phrase, more poetic language playing on the phrase "at the stroke of midnight"
A phrase is "at the stroke of midnight" which is a dramatic way to say at midnight, most often used when writing stories
We can also say "when the clock strikes 12" which is also pretty dramatic. "The clock strikes" (strike being a verb) means it plays a sound with the exact hour which old clocks do... I would say it is less common to say "when the clock strikes 8" or any other number, simply because as I said it is a melodramatic storytelling way to describe the time, hence usually at midnight... and not at noon! :) hope this gives some context to your question