Generally when we hear a whistling or whining noise in a pipe or fitting it is a direct result of an excessively high velocity of flow through that pipe or fitting.
In turn, a high velocity of flow can lead to much more serious problems than just a minor noise issue. First of all, as the velocity of flow increases the friction between the conveyed fluid and the pipe wall increases proportionally and as the friction increases there is a corresponding decrease in pressure. In addition, when the velocity of flow in a copper pipe exceeds 10fps(feet per second) we start experiencing pipe wall erosion, which can lead to pin hole failure. The same is true in a plastic pipe, such as PVC, CPVC or PEX when the velocity of flow exceeds 15fps.
For this reason the plumbing codes give us line sizing tables to insure the velocity of flow in a copper line does not exceed 8fps or 12fps in a plastic pipe.
Now in regards to your shower head. You stated that you have been in this house for 4years and everything has been fine up until about a month ago. From this we can conclude that the pipes must be sized correctly therefore we must consider what might have changed in the last month or so?
Sight unseen I would suspect that you have a mineral scale buildup in the shower head and that buildup . Either the effective working diameter of the shower head ports have been reduced by mineral scale buildup or some of the ports are completely obstructed, thereby increasing the velocity of flow through the remaining ports. Keep in mind that the mineral scale has been slowly building up over a long period of time but it has just now reached the point where it is manifesting itself by making the noise you hear.
In my house the remedy would be very simple, I prefer a very strong forceful spray in my shower so I would simply replace the shower head, but then my personal favorite shower head is solid brass and retails for $5 so replacing it is not a big deal.
If you have a more expensive shower head you might initially try using a needle to insure all the ports are open. If cleaning the ports with a needle shows some relief I would then remove the shower head and soak it overnight in a 50/50 solution of white vinegar & water, or use one of the commercially prepared solutions for removing mineral scale such as "CLR" or "LimeAway"