What would cause a rattle in the engine?

What is your question?

It kind of sounds like a rock polisher/tumbler. Runs fine, gauges read ok, nothing looks amiss under the hood, I just can't determine where the noise is coming from. It's been this way for the past few days.

What makes this problem better or worse?

Nothing

How long has your 2014 Ford F-150 had this problem?

A few days