Find out what is causing the noise by running the generator and listening.
Apply 10W50 oil to eliminate the noise of a piston slap. Rebuilding the generator's engine will also solve this problem if it is extensive.
Replace the lifters if the noise is coming from the valve area.
Replace the motor mounts by unscrewing and adding new pieces. This could cause a low frequency noise from rotted or dried out parts. This should stop a lot of the noise from the generator vibrating.
Replace the exhaust pipes so they extend past the body of the RV. Also, check that the exhaust pipe has no holes in it.
Apply a 3M dampening product if the noise is coming from the compartment where the generator is mounted. This will stop the vibration. Also, 3M makes a product used to reduce the noise of car doors. Spraying this product on the compartment box will quiet it down.
