2015 13" MacBook Pro crackle - not speaker related

A fan with a bad bearing can still run and cool the Mac just fine for a while. The fan could also be slightly bent where the fan rubs against the fan housing or perhaps some small obstruction could be lodged inside the fan depending on whether the tech actually removed the fan. And some fan failures can have a crackling like sound. Since the problem seems to happen as the laptop warms up, the fan is a good possibility as the fan may be increasing in speed which is why you are suddenly able to hear the crackling.

The fan and speakers are the most likely options. Some users have reported being able to hear a slight noise from the SSD, but the SSD is located more toward the left side. I doubt the noise is from the SSD since the only time I heard an SSD make a noise the sound was very hard to hear.

My money is on the fan.

Could still be speakers even if muted. Try running through this.https://homeexplained.com/how-to-fix-rattling-speaker-on-macbook-pro/ Does it only happen when first started up or continual?

It may be a bad fan.

I'm not going to take the bet. It's process of elimination. But fans can make all kinds of noises as they startup that may go away once they rev up. Speaker noises usually don't go away.