Insulation between floors - how affective?

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?