yes they are
Because the water heater is downstairs, and the pipes are poorly-insulated.
Answer The waste line from the downstairs toilet is not vented (or not vented properly, or the vent is plugged up). Flushing the upstairs toilet creates a vacuum in the main line which sucks the water out of the one downstairs. Answer your waste lines for both toilets may be on the same branch off the sewer main. Either you have a stoppage in the branch line or you have a stoppage in the main line. (If it's the main line your downstairs toilet would have to be the lowest fixture, ie If you have a floor drain downstairs and it's a stoppage in your main line it would backup through the floor drain. If there is no floor drain the toilet downstairs is probably your lowest fixture.) Your best bet is to snake out the system through the toilet downstairs.
The pressure of water in the pipes is called the 'head of pressure' and it is the difference in level between the water surface height (above sea level) in the nearest reservoir to the height (above sea level) of your tap. This 'head' will be higher for a downstairs tap than an upstairs tap and therefore the pressure and water flow rate upstairs will be less than for downstairs.
Anything connected to water pipes. Sink, faucet, toilet, shower and controls.
This is due to the gravitational force affecting the water flow. Water has to travel against gravity to reach the tap upstairs, resulting in a slower flow compared to the tap downstairs where water flows with gravity. Additionally, the length and diameter of the pipes can also affect the flow rate.
Your water lines aren't properly isolated. when you flush the toilet and your water lines are refilling with water it can create water hammer. This is usually worse if your water pipes are either not properly supported or come in contact with wood joists or studs. You may also have very high water pressure which means a lot of air comes through with the water, try closing the water inlet a little. A loose washer on the shut off valve for the toilet can cause the jackhammer sound you describe.
You have air in the system you need to bleed it out. do you have a really old heater/ boiler. hot water heater need more info
to your toilet
A toilet pipe cover is used to conceal and protect the pipes connected to the toilet. It helps prevent leaks, blockages, and unpleasant odors from escaping. Additionally, the cover provides a finished look to the toilet system and contributes to its overall cleanliness and efficiency.
To stop noisy pipes when flushing the toilet, you can try adjusting the water pressure, insulating the pipes, or installing a water hammer arrestor.
The noise from pipes when a toilet flushes is caused by the sudden rush of water through the pipes, creating vibrations and echoes that produce the sound.
old pipes in the sink or tub upstairs