I bought a surge-protecting power bar yesterday. It's a seven-outlet model, made by American Power Conversion Corp.; I found it at the thrift store for $4 second-hand. It appeared to be in decent condition, though I did drop it a few feet onto the hard floor shortly after buying it.
Last night, the house was quiet. I plugged the power bar into the wall outlet and turned it on. The "Protection Working" LED lit up. Good.
I then plugged my laptop's AC adapter into the power bar. As I did so, I heard a "pop". The sound was a bit louder than a finger snap, but quieter than a hand clap. My heart started racing.
Wondering if I damaged anything, I turned the laptop on. Thank God, it still works perfectly.
What could have caused the "pop" sound?
Is it safe for me to try plugging other things into the power bar, such as a lamp, to see what happens?
Should I continue to use the power bar, or should I discard it?
Edit: I experimented by plugging the AC adapter into a wall outlet, and unplugging it, a number of times. It indeed sometimes does make a spark and a "pop" sound. I plugged a couple of things into the surge suppressor. It's still working as expected. I'm keeping the surge suppressor. Thanks to Pigrew's answer, I now think the "pop" sound was normal. Pigrew, thank you very much for your answer; I've upvoted and accepted it.