The sound is similiar to a twin enging turbo prop aircraft where the engine propellers are not in synch with on another, making a harmonic oscillation type of noise. Other than the noise it works perfectly and has been for at least a month. Approx age 10 yrs old
Every AC I have ever owned has developed a rattle or some sort of noise after a few years or so. Mostly it was/is a buzzing sound. Some are quite loud.
It takes poking around inside to find the offending part, and usually some foam rubber or duct tape will fix it once you locate the problem.
One of my current ACs is one that you can’t get at the inside while it’s running. Every once in a while it makes a very loud rattling noise, but so far, turning it on and off has made it go away.