Champion sells a 3000 watt inverter model that is decently quiet.
Trick to having a generator under a pickup truck cap is placing it on a rubber mat to deaden vibrations, and to set up fans to keep air moving, because the larger generators will overheat without a lot of ventilation. I'd probably get a box fan and put it on the exhaust side, which will help disperse the CO gas, as well as pull in enough air to keep the generator decently cool.
As for noise, my 3000 watt Yamaha inverter generator is quiet enough that I don't hear it over the guy 100 feet away with the open-framed contractor generator. So, even though I paid about 6-7 times more for a quieter model, it has worked and worked well.