Hey There, What's That Sound....?

Just gone to bed - early morning ahead.  What's that sound?  Faint.  A pretty regular somewhat metallic tapping sound, a little resonant.

I couldn't imagine what it was - not a cat... maybe caused by the gusty wind... no that's not it.  The only thing I knew for certain was that it was the perfect kind of sound to make it impossible to fall asleep. Well, it can't stop me!

My first instinct with things like this is to try to ignore them - it doesn't make sense so it must not be important.  This may not always be the best instinct.

I did fall asleep.  I woke up about 3 am and could still hear the sound.   I got up, and as I walked across the quite room, I located the source of the sound as a heating system air return vent on the wall.  I walked around the house, upstairs and down, and could hear the sound every time I passed an air duct.
OK, it's 3 am, but I can eventually take a hint.  Down to the utility room.  As I approached, the sound was louder.  I opened the door and there was water on the floor.  Now the sound isn't the most pressing issue. I moved things out of the room that were in the water, got an old towel and a bucket, and picked up most of the standing water.

By then I knew the sound and the water were coming from the large ducting at the side of the furnace.   But where was the water coming from?  A leaky fixture upstairs?  A leak in the roof and rain blown in from the storm the day before?

I got a flashlight and made another tour of the house.  I pulled down the folding stairs to the attic in the upstairs bedroom and climbed up into the cold space to look around - no sign of water anywhere.

Back to the furnace room, where I went over by the ducting and thought I felt a cool mist on my head. I waved my hand overhead and indeed, there was a mist of water, but I couldn't see anything.  Back upstairs to get a step stool.  

Then I could see the source of the problem.  A copper pipe running between the ducting and the wall had corroded through and developed a pinhole leak, which was spraying a very fine mist out onto a large PVC pipe, where the water coalesced into drops and fell onto the side of the ducting, causing the regular metallic, slightly resonant sound.

I traced the pipes back looking for a valve, and concluded that the only way to cut it off was to shut off the main water supply line to the house.  So I did, then went into the laundry room and opened a faucet to relieve any pressure in the pipes.

Back in the furnace room, the mist was stopped, the drip silenced, the floor reasonably dry.

I went back to bed.  Ahh, quiet.  And a few hours to sleep before the first task of the morning - calling a plumber.

I really need to reconsider that first instinct of mine.