I've found a big difference between grinders, in terms of noise output.
I used to use cheapy ones, £10 each from toolstation (15 years ago...). Each one would last a year or two, but die within the warranty period so just be replaced.
Eventually, I ran out of warranty time as I stopped using them so much (yay!).
I bought a couple of Makita grinders, and there is a HUGE difference in the refinement and noise level. Yes, they're 5 times the price, but they are both nicer to use and quieter in running. The current ones have lasted probably 5 years (of occasional use) now, with no issues yet.
I borrowed a friend's grinder a few months ago, and really noticed the noise difference...
WHS ^ both my makita grinders are way quieter than my hitachi one - I can't even tell the difference in age between them and one is at least 5 or 6 years older
I have three grinders - one for cutting, one for grinding and one with a flap wheel- frequently I change one of my makita's to what I need rather than use the hitachi
What I really should do is buy another one but the hitachi hasn't died yet!!!!