Sounds like a loud leaf blower, no engine sound.

Dewalt D55168, 1.8hp, 15 gal oil-free upright. I performed the break in procedure when new and then it sat unused for about a year. My painter asked to use it with his spray gun, it fired right up and he commented on how quiet it ran. 2 days later I noticed 60lbs pressure on tank gauge and turned it on, it sounded like a loud leaf blower or shop vac and the pressure wouldn't build past 60lbs. I couldn't hear the motor like I did 2 days earlier. I drained the tank and tried again with the same sound. The local Dewalt repair center said that I damaged the motor by leaving air in the tank for 2 days? Quoted over $200 to replace.