Long story short, basement toilet wasn't in use for close to a year. After flushing it a couple of times, it failed to fill. Nothing was changed with the plumbing.

I removed all the hoses associated with the toilet and no water came out of the pipe coming out of the wall. The toilet is just a few feet from the water heater and the cold water line it's tapped into. They are separated by a wall. So you have the pipe coming from the main water shut off, going to the water heater, with the toilet drop some 8-10 feet before the heater.

Now, the part that has us stumped, the toilet starts filling a few seconds after the main water valve is shut off and then again a few seconds after it's turned on. Just replaced the fill valve earlier today in hopes of it working and nothing.

Another thing we noticed a few months ago is that if any toilet in the house is flushed, there's a single water hammer thump/whoosh coming from the sump pump or whatever that big hole in the ground by the heater is.

the toilet starts filling a few seconds after the main water valve is shut off and then again a few seconds after it's turned on. Just replaced the fill valve earlier today in hopes of it working and nothing. By main water valve do you mean the house shut off or the angle stop? What ballcock did you replace? (see image) Did you flush the lines out when you had everything open? After I find out what fill valve you have we'll open it up and flush out the lines. Back to you, Tom

The main house shut off. Got the valve on the left, 400a.
Yeah, all interior sinks were flushed after house valve was shut. Several times. Not sure about the toilets. Should the outside spigots be opened as well?
Thanks.

Should the outside spigots be opened as well? No! But I would like you to shut off the angle stop and twist the top of the ballcock up and out, (see image).
Remove them black washer, Now place a pan over the open valve and turn the water back on.to flush out the supply. Do you get a strong stream when you do so? If so then reassemble and give it a flush. OK now? Let me know, Ton

Nothing. No water coming through the angle stop whether it's on or off.

OK! That takes the ballcock and supply right out of the running. Now go outside and shut the water off to the house. Unthread the 1/2" chrome compression nut and pull the angle stop clear. Now place a pan over the open pipe and have someone turn the water back on for a few seconds. Are you getting water now? Let me know, Tom

Bit confused, sorry.
So, shut off the main valve coming into the house, unthread the 1/2 compression nut behind the toilet(one coming out of the wall into the angle stop), right, and turn the main valve back on. Is that correct?
But from what I noticed the past couple of days, yes, there will be water coming out for a few seconds. When the main is closed water will rush out for a few seconds then stop and when the main is opened, the water will again rush out and stop after a few seconds.

Either way, have the laundry running right now so that will have to wait.

Bit confused, sorry.
So, shut off the main valve coming into the house, unthread the 1/2 compression nut behind the toilet(one coming out of the wall into the angle stop), right, and turn the main valve back on. Is that correct?

Right on! I'm tracking back to where you lost your pressure. Good luck, Tom

Ok, so, no go. Got the compression nut off, turned off main water supply, it took 20-30 seconds for the water to start flowing from the pipe. It's slow because the water is basically draining from the pipes, right? When I turn the main house valve back on, the flow stops.

When I turn the main house valve back on, the flow stops. We have eliminated
1- The ballcock
2- the supply
3- And the angle stop.
That puts the problem in the system itself. I think we've checked everything a DIY could, Unless you know of a shut off valve on the branch you're going to need a plumber to go over your system. Good luck, Tom check,

I figured as much. Only shut off valves I see are the main one, another about ~4ft downstream from the main, and one right above the water heater.
Thanks for all your help. I really appreciate it.

Please keep me in the loop and let me know the outcome. Good luck, Tom

What does it mean when the water heater bangs after main water valve is closed?
Also, seems the water meter right after the main valve is slowly leaking. Would that cause any mishap in the home water system?

What does it mean when the water heater bangs after main water valve is closed? Installing a expansion tank, (seeimages) should eliminate your bang.


seems the water meter right after the main valve is slowly leaking. Would that cause any mishap in the home water system?Let's check and see, Turn everything in the house off including the main house shut off, Check the little pointer in the meter. Is it moving? No? Then open the main shut off to the house and check the meter again. If it's moving you have a hidden leak in your house. Check the toilets first and get back to me with the results, Good luck, Tom