Yes, 45db is loud. As a comparison, silent PC fans are around the 20db range.
Here is some info:
Dishwasher (Rinse) at 10 ft . . . 60 dB
Bird calls . . . 44 dB
The scale is logrithmic, so 60 db is a much louder than 44 db. But 45 and 44 are very close, so it is a good refrence.
Personally, it would bother me. Good news is the APC only runs it if it needs it.
I use two UPSs in my system. I have an APC SmartUPS 1500RM2U and an APC SmartUPS 750RM2U (rack mount models). I use two because of the startup surge caused by my system. The Plasma TV is on one and the Denon 3808CI AVR is on the other. The HTPC is on the one with the plasma, which is the 1500. The other assorted items are plugged in to either one, based on loading of the UPSs.
The 1500 has a fan, but it only kicks on if the room is warm and I have been having a movie marathon. Its fan is 46db at 1 meter, and it is too loud to have running during a movie.