By the spec sheet the RS2121+ should be both quieter and cooler than it predecessors and as a 2U unit with fans sized for space available should also suggest low noise too.
My experience misses the expectation above and I was surprised that I could hear it when I installed it in my rack. My rack is not overly noisy with a couple of UniFi switches, a couple of EdgeRouters, a Synology
RS217, Mac mini server, Lenovo mini-server and 2 large auxiliary fans at the top rear position, running at a slowish speed.
I would not say the noise is unpleasant or particularly loud - a 'rush of air' noise that is just enough to add to the overall rack noise. I'm not sure the fan curve selected by Synology makes complete sense either. There is negligible difference between 'cool mode' and 'quiet mode' and the amount of air pulled through the unit in 'quiet mode' is considerable and hardly warm on exit.
I do have a Synology 10GbE card in mine so that probably changes the fan curve and maybe Synology has just made a hash of it and let the fans run higher than they need to be. All the SSDs and HDD are reporting pretty cool on the slowest fan setting. I will add some drive-bay fillers to ensure the air goes where it is needed and that may help with the noise too.
Noise is subjective. If I close the door on the network cupboard I can hardly hear the rack. Walk 1m or so away from the door and I don't hear anything and it does not vibrate anything or cause a background hum.
I should add this is all set in a very quiet house in a very quiet area.