Re: noisy bathtub
As stated, if you have access under the tub, then you can reach under it and put in mortar or add to the mortar base. I'm not sure how hydraulic cement works, but I would be leery of using any expanding material, as it may apply too much pressure and lift the tub, creating a new problem rather than fixing an old one.
If you don't have access, then going through the wall from the opposite side is an option, particularly if you're going through a closet. If you are going through a visible wall, such as the living room, then the repair of the wall will always be noticeable because it's got a different texture, fewer layers of paint on it, etc.
I've installed mortar bases after the fact, and while not particularly fun, it wasn't a hugely big deal. Create a form around the bowl of the tub that is a little higher than the bottom of the bowl. Mix up your mortar to be on the wet/runny side, but not too wet/runny. You need it to be workable, not like moist sand. Work the mortar into your form until it overflows the form, this will insure good contact with the tub base. Let the mortar cure for 24 hours before using the tub.
Hope that helps.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!