Fan going off a lot

In that case begin with About fans and fan noise in your Apple product - Apple Support. One of the links it references includes this applicable passage from Keep your Mac laptop within acceptable operating temperatures - Apple Support:

"If your Mac laptop gets warm even when it isn’t doing tasks that require intensive calculations, or if it has fans that run for a long time, use Activity Monitor to check CPU activity. Look for malfunctioning processes that can put a significant load on the CPU and check for unexpected fan use. If issues persist, close and open the lid on your laptop, or restart your computer. ..."

Since yours are not running "for a long time" and become objectionably loud just once in a while I suggest keeping Activity Monitor open and running in the background so that you might have an opportunity to identify a particular app or process responsible. You can select Activity Monitor's Energy pane, or CPU sorted by %CPU to see which apps or processes are using the most energy or placing the greatest demands on your Mac's CPU, respectively. To show the most relevant information I suggest selecting "Active Processes" from Activity Monitor's View menu. Inactive processes won't be of interest for the purpose.

Ideally, capture a screenshot when this occurs: Take a screenshot on your Mac - Apple Support, and include it in a reply if you wish.

A Mac's cooling fins can become very obstructed with dust before the fan noise starts to become objectionable. That possibility grows more likely as a Mac grows old so I doubt that's the problem.

I understand your MacBook Pro's fans occasionally stop working. Apple Diagnostics will exercise the fans and verify proper operation: Use Apple Diagnostics to test your Mac - Apple Support