For past few months, I hear pipe knocking each time my toilet tank is filled and showerhead pulsating along with pipe knocking a few minutes into my shower. Turning off the water and restarting again stops the water pulsating. Then I recently noticed a moldy, wet spot on my ceiling that has progressively grown bigger. I also recalled that someone had left a plier at the water main valve into my home. I wonder if it's all related. Obviously I have a leak in my pipe somewhere between the ceiling sheetrock and attic decking. Can the leak be the cause for all the water hammer I've been experiencing lately? The toilet refilling doesn't use hot water. The showerhead pulsating does. Can someone explain to me what causes all these incidents in recent months? I've lived my home for 11 years now.