Funny you ask, it is loud but not in a cover your ear kind of loud. Today I was using the hotrod on stairs and at one point the resident peeked around the corner to see what the noise was and then he went about his business. It is just a high pitched noise that is not what we are use to hearing. Not a problem IMO. This tool is something I will be using almost daily, just easier on the back under certain situations.
I was concerned about the noise of the angle grinder. It is unusual to hear in the home so I was concerned about drawing attention with it.
I don't think it will matter and here is why I don't.
Last week I was cleaning with the Rotovac XL and someone had dropped something heavy on the motor cover. It dented it just enough that the fan would rub against the cover and clang loudly (it was awful).
This carpet while not overly soiled, it was pounded down from years of use and really needed the fiber lifting ability of an RE. I started out just going to use it in the master bath and bed room but it was cleaning so well and the client didn't even look over or ask about the noise. I cleaned the whole house with that racket. They were thrilled with the results afterwards and could believe how great it looked. I was just glad they weren't deaf afterwards!
I took the cover off and tapped the indention out when I got back to the shop.
So the noise factor isn't at all what I thought it might be.