Can't figure out the origin of loud screeching noise
You are describing brake squeal caused by the brake pads resonating on the rim during braking. Wet rims often squeal worse than dry ones and the flour use reduced the friction somewhat which reduced the ability to resonate as well as diminishing brake effectiveness.
There is a way to alleviate much of the braking noise with the technique of &quot;toeing&quot; in the brake pads very slightly so that the front of the pad contacts the rim first and then the rest of the pad closes further. This helps prevent resonating which is the cause of the noise.
A rubber band wrapped around the trailing edge of each pad will provide ample toe in when the loosened pad, brought into contact with the rim by activating the brake lever, is then tightened down with the pads fixing bolt. A business card placed between the trailing half of a brake pad and the rim is an alternate method of setting the toe in.
Additional actions that can be taken are cleaning the braking surface of the rim with alcohol and a scouring pad to remove oils and other contamination that can add to the tendency of the system to resonate.  At the brake pad, use sand paper to rough up and clean the braking surface of the pads.  Pads develop a hard glaze with age and the heat from braking which can contribute to noisy braking. Sandpaper removes the glazing exposing softer rubber below. The roughened surface of the pads contributes to the friction potential which can change the ability of the pad to resonate on the rim.  Utilizing a softer compound pad can be of use as well. Kool Stop is a popular, quality brand of brake pads that many bikers swear by.  One can acquire brake pads that have a longer contact patch on the rim--the pads are essentially longer which effects the resonation potential as well.
Park Tool has an excellent instructional video on linear pull, &quot;V-brakes,&quot; such as yours, and includes the toe-in technique described above.