We recently renovated a 1920 craftsman that had hot water radiator heat. We put in a new furnace 2 zones. the LR< DR
The idea that hot water (hydronic) radiators and hot water baseboard can't be mixed is an old wives tale that is still with us after all these many decades---take it from me; I've had the SAME MIX OF RADS & BASEBOARD THAT YOU DO in my home for the past 40 years and the rads and the baseboard get along quietly & very well, producing good heat without any noise whatsoever!
The old wives tale for hot water heating began in the l960s when, up till then the convectors were all hot water cast iron radiators; with the introduction of hot water baseboard in the 60s the false rumor spread around that the rads and the baseboard were incompatible---not true!
Just about all "noise problems" with hydronic baseboard has to do with improper installation procedures where insufficient clearance holes for the supply & return piping (usually 3/4" copper or steel) causes expansion & contraction when the hot water from the boiler courses thru the piping for each radiator---since the supply/return holes in the sub-floor (rafters) are OFTEN TOO SMALL, the piping whack against the flooring/sub-flooring as the heated metal expands, creating the objectionable noise.
The "loud rapping sound" overnight that the woman complains about is a classic description of the metal supply/return pipes (that lead to & from each of the baseboard ) that are banging up against the supply/return holes in the floor/sub-floor---metal supply/return piping expands when it is heated up as the hot water is pumped from the boiler----since the baseboard elements themselves & supply/return pipes are usually made up of 3/4" copper piping, the expansion & contraction will be greater than the cast iron rads, which also have supply/return copper or steel piping-----the noise is much more noticeable overnight when all else is quiet.
The solution to the problem will often entail widening the supply-pipe holes in the floor/sub-floor at each end of the baseboard elements, or installing neoprene inserts in the existing holes; sometimes it is enough to inject some "high heat" caulking compound (Home Depot/Lowes) into the supply/return holes; the wider use these days of semi-flex plastic PEX tubing for the supply/return lines (instead of copper or steel) is another possible solution; quite often the boiler repair person will cut out approx 12" of the offending supply/return metal piping that's making the noise & insert a 12" section of flexible PEX (plastic) tubing in its place with permanent clamps---this will serve to ABSORB most or much of the heat expansion now being experienced with the metal supply/return lines, & thus eliminate the noise.
The thing to do is to call your local boiler service man to check out the problem and for him come up with a solution---sometimes if the boiler water operating temperature is set somewhat on the high side (180-200 degrees) this will cause more pipe expansion & knocking noises than if the boiler repair man reduces the boiler water operating temperature down to perhaps 165-175 degrees---this lower temp is completely adequate to heat the baseboard, but will often lessen or eliminate the noise.
A similar noise in steam systems has a different cause. (This doesn't apply to you since you have a hot water system, but others with steam systems may read this, so I'll point it out here.) Of course, noise in steam systems can have the same cause -- expansion and contraction of improperly supported pipes -- but there is an additional cause.
The supply (and return, if so equipped) pipes of a steam system must be properly sloped and supported. Think of it this way: if you opened up the pipes at the boiler, any water in the pipes should drain COMPLETELY out.
A sag in a pipe, or an improper installation that results in a sump or weir (a dip in the piping like the trap under your sink) will trap condensed water. This water will block the flow of steam, until the steam reaches sufficient pressure to burst past the condensate. When it bursts past the condensate, the effect is a loud BANG! heard throughout the house.
If the noise happens both during heating and cooling, it's probably due to expansion and contraction of the pipes. If it only happens during heating, you've likely got a sump that's collecting condensate.