This could be a few different things, from as simple as sitting too often, an injury to the area, to a varicocele or spermatocele. Since it's new for you, you'll want to get it checked out. It's probably nothing serious, but it may be something that needs treatment, especially if it lingers.
I'm including some older threads that are similar, and were answered by doctors when we had them on the site. I hope they help.
https://
www.medhelp.org/posts/Mens-Health/Testicle-fluttering/show/1632362
https://
www.medhelp.org/posts/Urology/Pulsing-or-throbbing-sensation-in-testes-area/show/988457
https://www.mayoclinic.org/diseases-conditions/varicocele/symptoms-causes/syc-20378771
https://www.healthline.com/health/mens-health/spermatocele
Let us know what happens.