This is a weird one folks -
I had a new bathroom built. I can now hear water flow in the hot water lines throughout the house. I know I have a problem and I have narrowed it down, but here's the question:

The new tub has a single handle mixer valve. If I turn off the cold water supply to the water heater, the tub will flow cold water normally, but not hot (as it should be, right?)

However, if I turn off the hot water supply coming out of the hot water heater, the cold water flow at the tub is just a trickle (hot is off, as it should be).

No other fixtures in the house are affected. Turning off the hot either before or after the water heater shuts off hot and allows cold to flow normally.

The house is slab on grade, so I can't go under & look. I have tried to draw a schematic of how turning off the hot water supply at the heater can affect the cold flow, but for the life of me, it's got me stumped! I'm about ready to think I'm demon possessed & call for an exorcist!

Any thoughts?