Insulation between floors - how affective?

I feel your pain as my gas boiler is right next to my dedicated listening room in the basement. Sucks. See the link below for Premier Home Theater by Earl Geddes and you’ll get some very approachable and effective options, specifically in chapter 11. My knee-jerk response would be to install two different thicknesses of Sheetrock bonded together with Green Glue and maybe even install resilient channels — depends on how serious you wanna get about this, but those are only two of many ways to attack this. Best of luck.

http://www.gedlee.com/downloads/HT/Home_theater.pdf

Check first that this isn't a routine maintenance issue. Noise from circulating water could be air in the baseboard piping. You'll need to bleed and fill. Rumbling noise from a boiler could be a restriction in the chimney or flue, caused by soot (sort of like clogged arteries). Maybe time for a chimney sweep. 

I sound proofed the air compressor I use for my tonearm by using these sheets of melamine foam with mass loaded vinyl in between-- that stuff is not acoustic treatment it is sound proofing material. It is expensive though. The mass loaded vinyl is like the plastic equivalent of lead. Melamine-- better known as the stuff that is sold as Magic Eraser--was originally developed for sound proofing. I have used this material in a couple applications. I think it would be far more effective than "insulation." BTW, the cabinet in which I installed the compressor (and which is lined with the melamine/mass loaded vinyl) is made of marine ply- two layers- with green glue. Crazy heavy. But dead quiet. I would look at sound proofing topics since I think that's the category you are in-- maybe there are cheaper alternatives. 

Erik +1

I'm concerned the sound maybe radiating up along the side of the structure and vibrating a wall that transfers sound into the room.  

Is the pump touching the wall?