That Onan Model number doesn't register anything when I google. Given the age I suspect its an ONAN 4000 BGE which I own and isn't that loud. If so - It's not as quiet as some of the newer larger high end models but it's a rock solid model that isn't noted for being extremely loud. It's certainly not as quiet as a Honda or some of the other models but while I don't run mine often in campground I have never had a complaint.
Some purchase a muffler extension baffle - haven't needed it myself. I have read post where people put material on the ground where the fan exhaust (don't recall what type of material .. but you can use the forum search feature) ... that's suppose to reduce the sound.
Hope this helps.