right when it turns on it pops? that sounds like it could be a grounding issue in the microwave. i would honestly advise against using it if you hear electricity popping like that when you power it on. im not very savvy on repairing household appliances but i DEFINITELY wouldn’t use it anymore until properly repaired.
if youre feeling adventurous you could
unplug it and let it sit for a while for the residual electricity to dissipate, then open it up carefully and see if you notice any scorch marks on any of hte PCBs or wires in the unit itself.