Modern boilers are rather quiet, except for combis which fire up with a large flame and fast fan speed, especially for baths and showers.
IMO the two most important things with a new boiler are:
Must be close to an internal drain, such as a sink waste, so the condensate pipe can be plumbed in without going outside. Apart from being unsightly, external pipes are prone to freeze in cold weather, which is exactly the time you don't want your boiler to stop working
Must be in a place where the steam plume will not blow past your windows or shroud the front of the house in a cloud in cold weather. Again this is worse with combis.
It must also be close to a gas pipe and a water pipe, but that is not difficult to fix.
Don't put it in the loft. It will be tiresome to get to, and unless you have a fixed ladder with guard rails round the hatch, good lighting, and a safe floor, many maintainers will refuse to service it. Lofts are also very cold in winter, so it will keep firing up to prevent itself freezing.
If you have more than one bathroom, consider an unvented cylinder such as the Megaflo brand. It can provide unsurpassed hot water.