I know this is an old thread, but thought I'd contribute anyway. I experienced a similar scenario. I was standing in my kitchen and saw a bright, electric flash at the bottom corner of my dryer while it was running. The dryer did not stop running in process, and it seemed to still be heating fine. Since I was watching it while it happened, I saw that the flash came from the area where the heating coils are. I opened up the back of the dryer, took out the heating coils, and sure enough, there was a nice sized black spot. The funny thing is that the coils still had continuity and the dryer is still heating. I'm not exactly sure what caused the heating coils to spark, but seeing the burned out spot on the heating element at least gave me assurance that I didn't have a short in my wiring.

On Thursday, November 14, 2013 at 3:44:17 PM UTC-5, Danny D'Amico wrote:
The wife said the dryer was sparking "in the back".
I moved things around, but didn't see anything so I tell the wife not to worry.

A week later, I turn on the 220v 3-wire dryer, and a single bright white spark
snaps in the back, between the dryer and the 4-foot 6-inch diameter aluminum
corrugated lint pipe connect to the outside vent.

Huh?

Could it be static electricity?
Could it be the dryer frame is hot?
What do I test?

The only connection to the wall is the three-pronged 220v dryer cord, which
looks perfectly fine when I remove it from the wall outlet.

There is no specific "ground" wire from the dryer to anything (should I have
put a ground wire in to the 120 volt outlet nearby?).

What would you test?
If you need pictures, just ask.