Either you don't have the knob pulled all the way, or the disc in the spout is faulty.
Of course it leaks as water has to drain from the shower riser
Employ a plumber.
Is more water comming out your tub spout? if so change spout
if its just a shower and no tub spout then it most likely must be in your shower head and if its a hand held shower head (with a flexible hose attached) it most likely has a bad seal or some other way which allows air to get inside and drain the water out slowly. The same can be for a shower with a tub spout diverter and the shower head has a bad seal thus letting the water drain down the tub spout and it can be a real slow leak to cause the squeaking noise 20 minutes later, my guess of the squeaking noise would come from the air being pushed inside the shower head.
The valve is not be diverting all the water. This valve may need to be replaced.
depending on your spout diverter if its a valve or is it on the spout it self, if the diverter is on the spout then your diverter is not letting full flow of water out of the spout and its putting enough pressure to allow the water out of the shower head, I had this problem before and if you change the spout it should fix the problem. If you have a 3 way valve which is on the wall then i would suggest replaceing the parts in that valve if possible other wise change out the divert valve. If your broke like me and want a cheap fix then put either a water flow resistor or a shutoff right before your shower head.
More "VOLUME" flowing from the tub spout then the shower head as the shower head has a flow restrictor
Your tub spout isn't fully diverting the water to the shower head and a very small amount is leaking through. The smaller the hole the water has to exit, the greater the pressure. Replace the tub spout, usually costs less than 20 dollars.
May be build up in the valve itself or the shower head.
To drain the water that is in the pipe or spout..
Its a loss in water pressure to the shower.
Probably rust and minerals that came loose when you turned the water off and plugged the screen in the spout. Unscrew the screen the the end of the spout and see if that doesn't fix it.
The whales spout water every so often. The water spout is clogged.