It is normal to have much higher flow from the spout than from the showerhead. If you describe the valve assembly in more detail the source of the noise may be more easily determined. It could be a loose washer or a faulty pressure balance spool.
Yes, roughly 4 gpm vs. 2 gpm is normal. Numbers will vary with type of shower head and system pressure but this is the ballpark difference. Flow restriction/pressure compensation is usually somewhere in showerhead assembly. (When I had ones underperforming specs I actually used a very small drill bit to fix the problem in some designs.) Flow in my current Moen shower valve doubled when I tested without the showerhead.
Moen made/makes a lousy version of a low flow head that had too many holes (roughly 48) in a poor pattern. They made for very unsatisfying showers with no real impulse to the stream. As a result we showered longer trying to get comfortable, without success. I replaced two Moen showerheads with low flow Evolve showerhead that I used in my old home (this one, original Roadrunner has 36 holes, newer type Roadrunner II's have much more...might not be a good thing but I haven't tested the new ones.) The whole family has been thrilled to have the "old" showerheads back again. These older Roadrunners were non-aerated, but had good velocity. Spray pattern design is critical and I like theirs. I've had to do this sort of layout frequently for plant equipment design in reactors, distillation columns, demisters, and spray cooling belts and am picky about even distribution in my designs. Many process equipment designers don't appreciate/consider the geometry of proper flow distribution and their designs underperform as a result.
Non-aerated designs don't lose as much temperature/heat before they hit your body (roughly 5 F + degree difference.) They also don't steam up the room as rapidly as a result. Non-aerated showerheads are more thermally efficient, but need to be paired with a tight design to give a good shower. Aerated designs provide an air/water mix that impinges more at a given flow rate, making for a more vigorous shower in a good design, but sometimes this is too much of a good thing... and aerated designs are inherently less thermally efficient.