I use an 800-900w generator that I bought from Harbor Freight Tools for $129.00. It's not loud at all, and you certainly can't hear it over all of the gas and Nitro plane engines running.
I put it in front of my van and run an extension cord to the rear. With the gen in front of the van, it blocks nearly all of the sound of it running.
I'm running 2 chargers off of it at all times, a Triton EQ, and a Turnigy Accucel-6, both of which are 5 amp max charge rate. Running the generator constantly, 4-5 hours, running both chargers, it only uses about 1/2 a gallon, so no need to carry a gas can. It holds 1 full gallon of fuel mixed with 3.4 oz of 2 stroke oil.
I usually take 3-4 planes with me when I fly, and the packs are constantly being recharged. I love the setup, and nobody has ever said anything about the noise. The Honda generator that is comparable in size to mine is a bit quieter, but expect to pay $400.00+ for one. The one I have is about 1/2 as loud as a regular weedeater, so not too bad.
Where I park my van is only about 25' or so from our pavillion where the pilots and spectators will hang out and chat, you can't hear the generator running.