Unfortunately you cannot throttle a water main valve in most cases as it a a gate type of valve rather then a globe valve and most globe valves have a washer and this is not a good thing on a valve you cannot isolate.
What you should check is your incoming pressure and your pipe size as you could have excessive velocity making this noise.
The noise is a warning that you could possible have possible erosion of this copper tubing taking place.
If you get the tubing size and the pressure ILL be happy to give you your velocity and we can then take steps from there.
Are you on well or city water? Do you have a pressure reducer on your main? Was any piping recently replaced?
Look for some very sharp bends or excessive use of 90 degree fittings.
There are lots of factors that can cause this type of condition, some are quite simple to find others are never found.
Roger: Whistleing and screeching in copper piping can be the result of excessive velocity of the water whizzing through the piping or in some cases the result of poor installation practices by the plumber. Failure to adequately ream the piping prior to soldering the fittings in place.If the noise is really bothersome, you may wish to have a PRV (pressure reducing valve) installed in the supply line to your home. By reducing the pressure, you can lower the velocity of the water, it will be quieter and the piping will last longer as well. Lots of luck Bud