Hi and Welcome to Just Answer,
Ok, let's take a look at this;
No, that is most likely the new parts, or dust, metal shavings, and so on that he cleaned and serviced.
The metal is expanding and contrasting. After a little while the burner will become seasoned in a matter of speaking, and stop.
And if it is the dirt and dust, same thing, after it runs a while it should stop.
Some may even be coming from the ducts, since your heater was working only half of the burners, then it is running and producing more heat now
If you need more help Let me know
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