Something weird.
I went to change a couple of plug wires and on the #7 coil and plug(5.3l 2014 Silverado 1500 4x4) when I pushed on it to connect I heard a hissing sound. Truck,of course, not running. It was like air pressure built up. Anyone have a clue?

 

Maybe air escaping around the tight fitting as you compressed it

 

The funny thing is ,is the boot was on. And it only did when tugged a certain direction. And it was a lot of air. The engine was warm so maybe it constantly built air pressure somehow. The darndest thing ever.

 

Maybe a loose or cracked plug perhaps.