Bleed it out
Your drain line is plugged somewhere after your shower and toilet line are connected. The reason for the gurgle in your toilet is because air is trapped in the drain and your toilet is the closest place for air to flow to. If you had a vent on your toilet trap arm then it would gurgle in the vent rather in your toilet but your drain is still plugged and needs to be snaked out and maybe your drain line needs to be examined for right grade or broken.
Yes. You need to open the plumb, & you have an air vent there.
Yes, the toilet is leaking from the water line.
A toilet may bubble due to a clog in the drain pipe, a malfunctioning vent pipe, or a problem with the sewer line. This can cause air and water to be trapped, leading to bubbling when the toilet is flushed.
Is the water line in your toilet leaking in the bathroom?
What is the recommended air change for a toilet?
I reckon there is no air vent pipe on the drain line of the toilet you are flushing and the siphon action is sucking out the water in the loft toilet also.
This could be caused by either a clogged vent line or lack of vent to outside atmosphere. It sounds like your drainage system is starving for air and is trying to pull air from the path of least resistance, the toilet. Also, it may be caused by the drain line itself being partially clogged
The purpose of the water supply line valve in a toilet is to control the flow of water into the toilet tank, allowing the user to fill and flush the toilet as needed.
To replace a toilet supply line, first turn off the water supply valve behind the toilet. Disconnect the old supply line from the valve and the toilet tank. Install the new supply line by connecting one end to the valve and the other end to the toilet tank. Turn the water supply back on and check for any leaks.
The standard toilet supply line size is typically 3/8 inch in diameter.
Your toilet may be bubbling when flushed due to a clog in the plumbing system, particularly in the vent stack or drain line. This can cause air to get trapped and create bubbles when the toilet is flushed. It is important to address this issue promptly to prevent further plumbing problems.