Well, here's my marginally-educated guess: the thump after flushing is most likely due to 'water hammer', which can happen whenever there's a sharp change in water pressure, such as flushing a toilet. The fastest/cheapest way to address the issue is to reduce the speed at which your toilet refills, by cranking the supply valve (usually at the wall, near the toilet) closed a little bit. So your toilet takes twice as long to refill, big deal.
The 'real' way to fix it is to add 'buffers' in the plumbing. Home Desperate sells special fittings, but you may already have something similar. If the plumbing runs through the attic, you're likely to see 'stubs' sticking up here and there, just capped-off pipes. Once upon a time those pipes had a pocket of air inside, and the air acted as a shock absorber to reduce water hammer. The only way to get the air back into them is:
- Shut off the water to the whole house.
- Open the faucets and hose bibs to drain the water.
- Take the caps off the tops of the 'buffer pipes' in the attic.
- When the buffers have no water in them, put the caps back on tight.
- Close the faucets and hose bibs.
- Turn the main water back on.
- Open the faucets one at a time, to flush the air out of the system.
Now the 'hissing' sound may be another problem. You could have a leak which is allowing air into the system. It's possible the colder weather caused some shrinkage somewhere, creating a leak. The 'pressure regulator valve' theory sounds equally reasonable, although I'm not sure all houses have one. I've been all under my house and never seen such a thing: I'm pretty sure my water pressure is just whatever the City decides to provide that day...
c.
