I asked the same questions when I was looking for generator...basically is the sound additive and the answer I got was the sound is somewhat additive but not very much. If you look at the specs for the 3000 it is 49db at idle and the 2000 is 53 db at idle.  If you have ever heard a 3000 that is running at slow idle it is extremly quiet, the 2000 is also very quiet at idle but the 3000 is almost unbelievable.  Both are around 58 at full throtle still very quiet.  I have had my double 2000 set up for one summer and a lot of people have walked by my generators and commented on how quiet they were.  I guess all of that is a long way of telling you that while the parallel 2000s may make a little more noise than the 3000 both are going to be so quiet that the difference in sound should weigh very little on your decision as to which generator to go with.

Sorry ... sent before I finished..  Yes two eu2000i will be louder than a single Eu3000is (Not the Handi tho)

From this site...  http://www.hayesequipment.com/honda/generators/honda_super_quiet_generators.htm

eu2000i :  53 -59 dBa

eu3000is : 49 - 58 dBa

I think one thing is being overlooked, when people are considering the amount of noise the various generators produce. This Honda, for instance, is given a DB noise value at 1/4 of its rated output. In addition, its fuel consumption is also measured at this lower value. When these generators are under load, which is why they were purchased in the first place, they will not be sitting there running at an one quarter idle. Under any normal loading, they will be burning much more fuel, and making more noise.

When you compare the specifications of the various manufacturers, make sure the ratings are compared equally. It would be a stretch, but someone could say his generator used no fuel and is absolutely quiet...until it's started.

I'm familiar with Honda's "Eco-throttle" and that's exactly my point. Consider Honda's EU30001. It is a 3000 W generator that is advertised that it will run 7.7 hrs on a tank of fuel, produces 57 DB and weighs only 78 lbs. Impressive figures, but they are attained at a 1/4 throttle which is not enough to power a microwave, air conditioner, coffee maker, hair dryer etc. When the generator is run at its available power, the 3000W drops to 21.7 amps, the run time drops to 3.6 hrs, and the noise jumps to 65 DB.

Compare that to a Yamaha EF 28001 which has a different rating system. It is advertised it can run 15 hrs at half throttle, and from that power setting to a full run, has a noise rating of 60-67 DB. It too is a 3000W generator weighing 66 LBS, but produces a  continuous rating of 23.3 amps. It too has a variable RPM depending on the electrical loading called "Economy Control".

I am not promoting one generator over another or trying to change anyone's buying habits. I am trying to show, that the initial claims of features have little to do with real world performance. By digging into the specs of a working environment, you can then determine which generator has the better features. Then if you compare pricing, you'll find that one lists for almost $1000 less than the other. It pays to shop.