A long time ago in the tropics...as a 76 new co-pilot new to the area flogging along in moderate rain with a benign radar picture well clear of any nasty stuff....suddenly heard a rapid "tick, tick, tick" in the headphones...asked the Capt ( a great guy with lots of time in tropical areas ) "What's that ticking noise?". His reply was "Oh Crap!!" this was followed by a big pink flash/bang so I followed his instructions. He told me afterwards that that strange fast ticking often preceded a close lightning bolt. Strange but true.