Our Coleman Mach 15 (15000 BTU) has two settings: Extremely Loud and Even Louder Yet.
Now, if something is out of balance, like a load of dirt or a nest in outdoor fan or indoor blower wheel, it can add noise you can reduce with a cleaning.
Also, if the sealed system is rubbing on part of the A/C, that'll add noise to the mix. Our first Coleman was a Mach 3 (13500 BTU). It was noisy and I found a problem with the rubber cookies the compressor rests on. Too long to remember exactly what was wrong, but basically it was going metal to metal when it should have been riding on rubber. And I was able to get it back in its proper position and back onto rubber. I also found some of the wiring was rubbing on the compressor and the insulation was about to go through. So all in all the noise was a GODsend.