On older models
Behind the valve stem sleeve and escutcheon there is usually a packing gland nut which can be tightened with a valve stem socket wrench designed for the make and model of shower control your asking about..
The Idle Air Control valve is fastened to the top of the throttle body with 2 bolts that have an 8mm head. Tighten them clockwise.
The valve in the shower control needs to be renewed.
You have the guts out of the control correct? Re clean the parts, flux and heat the body of the control.
If you mean shower valve, screws are for shower valve coverplate.
Your water pressure is low. Consequently, when cold water is diverted to another application such as a toilet or a sink, the amount of cold water available at the shower mixing valve decreases as the low pressure is unable to keep up with the shower's demand. This causes a hotter mixture at the shower head. The solution is to either improve the cold water supply to the shower or to replace the shower control valve with a temperature compensating type. Pressure balancing is the cheapest and simply changes the shower temperature according to the pressure in the cold and hot water lines. When the toilet or sink is used, reduced pressure to the shower valve is detected and the valve reduces the hot water pressure, thereby maintaining the temperature. The more sophisticated approach is a thermostatic shower control valve. These are significantly more expensive, but will not reduce the pressure at the shower head (a drawback of the pressure balancing valve). The thermostatic valve would be required when one shower control valve is operating several shower heads. Moen, Kohler and Delta all make both types of control valves. They can be readily obtained from a local plumbing supply house.
Behind shower valve handle and trim plate
Thermostatic shower valves were designed for safety, which allows the regulation of the water to within a half of a degree of the selected temperature. The most common thermostatic control is a single - handled 3/4 valve.
You replace it. If it's a squeal, you replace the shower head fist. If that doesn't make the noise go away, you replace the shower control valve. Odds are since you came online to ask how to fix it, you're not qualified to replace the control valve. Call a plumber now or call a plumber and a drywall contractor to fix it after you make your "repairs"
No. A little confused by the description of a threaded 90 and having to cut the valve free. Every shower I have ever done the final connection to the valve goes on with a nut. You should be able to take one side loose and leave the valve in place. Disassemble to the leak and replace.
If the noise is only for a few moments after taking a shower, its the expansion on the hot water side. Water lines going through studs will expand and make noise. the only way to fix this is to get in the wall and either loosen the pipe or tighten it with a shim.
The incoming hot and cold lines go to separate inlets on the mixing valve. The single output will go to both the spigot and the shower head.
Cut out the drywall in the wall behind the shower. Replace the valve. Patch the drywall.