For several months now, when we run the hot water in our bathroom faucet for about 30 seconds, we hear a "thunk," which is followed by about 50% pressure loss. If we turn the water off for a moment and back on, the thunk happens a few seconds in, and pressure loss occurs again. Recently, it began happening with our kitchen faucet as well. The thunk sound comes from somewhere near the hot water tank; is this an indication that something is wrong with the tank?

Thanks.

-Glen

Not necessarily, its more of an indication that you have something in your hot water line that is moving and suddenly partially blocking the line and reducing the water flow. When you turn the faucet off what ever is blocking the valve falls away. When you turn the faucet back on the water flow again moves it to the valve port. The sound is probably coming from the area of the water heater because that is where the pipe can move the most. My favorite thing to blame for this phenomena is solder inside the pipe (assuming you have copper pipes). When soldering pipe small amounts of solder can run inside the pipe. If this solder falls to a cold section of pipe it does not stick. It can lay dormant for a long time until finally the flow of water moves it to the next available valve. There it gets stuck because the valve port is to small for it pass. It doesn't happen that often but it does happen. It's a bit of a stretch to say that you have it happening in two faucets at the same time but it could be. Maybe you had a really sloppy plumber that used too much solder. The remedy is to back flush the valves and attempt to clear what ever is blocking.
If back flushing the valve does not solve the problem the blockage is probably in the stop valve. Turn off water, remove stop valve, clean input side port or replace stop valve.