Hot or cold?
Pressure certainly does increase in a water line going down hill. This is because the weight of the water pulls it.
water pressure goes down because the force of the water going through the space decreases
When you add a teaspoon of honey to water with vapor pressure, it will reduce the vapor pressure. The sugar in the honey leads to the pressure going down.
This depends how you are perceiving this "pressure going up and down" . I've repaired and replaced toilets for years and couldn't imagine how I'd gauge pressure doing this. Maybe you are mistaking pressure for flow rate and something is plugged up somewhere in your toilet.
what are many have air pressure if the one drum is going up and other one is going down
if the water and oil levels are fine, then you may have a sensor going out
There is pressure in all water because of the weight of the water above it pressing down
Water pressure in the ocean is caused by the weight of the water above pushing down on the water below. The deeper you go in the ocean, the greater the water pressure because there is more water above pushing down. This pressure increases at a rate of about 1 atmosphere (14.7 psi) for every 10 meters of depth.
The water in the sink may not be going down because there is a clog in the drain, preventing the water from flowing through.
PRV
Water pressure increases with depth, as you go deeper there is more water above weighing down on you.
Drains make noise when the water is going down because the water hits the drain as the water goes down. The water also is going down at a rapid pace, that makes the same sound as a waterfall. When the water is falling to the direct drain, the water beats on the drain.