Sudden loss of water pressure

We've experienced a sudden loss of water pressure in the master bath of our California ranch-style house. Slab construction. The problem is confined to this room only. All of the other fixtures in the house are functioning normally.
The MB is part of a recent remodel. Half-inch copper pipe is about four years old. This bathroom is supplied by a dedicated water heater. (another water heater supplies the rest of the house.) Fixtures include a vanity sink and faucet, soaking tub, separate shower stall, and commode. The commode does not seem to be affected although it is difficult to tell. There are no visible leaks in the walls, ceiling, at any of the fixtures, or the water heater.
The problem began quite suddenly on Sunday morning. When one of the fixtures is turned on, the pressure seems normal at first, then we hear a "thunk" in the wall in the general vicinity of the dedicated water heater (located on the other side of the wall from the soaking tub), and the pressure drops to less than one-third of normal.
The problem is also intermittent. Occasionally we can turn a fixture on and pressure will be normal. No "thunk." This occurs perhaps once out of every ten times. The remainder of the time we get the symptoms described above.
Looking for a place to start troubleshooting.
-Frank
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Does it happen to both cold AND hot ??? The obvious place to start is the water heater that goes THUNK....Clogged up or air bound maybe ???
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Frank Warner wrote:

There may be a water pressure regulating valve that's faulty somewhere in the line.
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During a recent bathroom remodel I got a chip of ceramic tile in a pipe. The cold water would suddenly all but shut off.
Jimmie
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As the other poster said identify if its hot or cold. Then try shutting off the feed to the bathroom, then open up all faucets, and drain them at the lowest point of the house ( basement). If indeed something is lodged in there, it could be dislodged doing it this way.
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I've narrowed the problem to the ball valve feeding the water heater. Holding my hand on the valve while someone turns the water on in the bathroom yields a noticeable "thump" at the valve and sudden loss of pressure. Turning the valve on and off a few times often brings pressure back up.
I suspect either an obstruction in the cold water pipe upstream from the valve or a faulty valve. Solution will be to remove the valve, flush the line, install a new valve.
Will let you know how it turns out.
-Frank
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