Knock the amp and you'll probably hear a ping. I've heard it on mine. Some guitar tremolo springs do too (now that might indeed be a problem).
Anyway, lot's of things with springs ping, even furniture.
Oh no, I just released a spring-loaded pen's button and it pinged. I must return it at once, it's clearly defective. Even better, we need a class action lawsuit against spring manufacturers. By law, all springs should come rubber coated, or come with a silicone core, or change alloy every 2mm of wire (Soluter's solution to avoid the resonance).