Strange soldering phenomenon

[KT]

I experienced a strange phenomenon when I was putting together my Brian GT gainclone kit recently. I was wondering if anyone has an explanation.

On two or three solder joints, I noticed that as I was applying solder to the heated pad and component lead, there was a small but distinct "pop" or "snap" coming from the solder (or the joint) as the solder melted into the joint. It sounded like a tiny, tiny firecracker. The solder filled the joint with no problems and the joint looked fine.

This was the first time in my 27 years of soldering that I experienced this particular thing.

I surmise that it might have been an irregularity in the solder - perhaps an irregularity in the flux that resulted in a rapid expansion of gas, and thus a small pop, as the solder quickly heated up and melted.

It's a trivial thing, but kind of curious. Has anybody seen this before?

At the time I was using a Hakko iron and thin gauge Wonder Solder.

Best,
KT

[Stocker]

cooking flux. flux steam. not the best to breathe, not too harmful for the joint if it comes out not-cold.