There are three main types of noise you can get from HW baseboard heaters: 1) noise from the differential expansion/contraction of the copper pipe and aluminum fins during heating/cooling; 2) air in the HW lines; and, 3) water hammer when the zone valves close. There is little you can do about the first, except to solidly anchor the pipes, radiators, and baseboard housings. To minimize #2, the pipes need to be bled off when you fill the system and you must install an air bleed-off valve, which is usually placed atop the special tee (actually its a “quad” fitting) above the expansion tank. (You do have these, right?). #3 is a variable problem that may appear if the system is poorly laid out, the HW pressure is too high (which it shouldn’t be if its regulated properly), and may be more likely to occur with certain brands and sizes of zone valves. One thing that can be done in cases of water hammer is to install a water hammer arrestor right after the zone valve. I don’t routinely install these, but in a few cases they have eliminated the problem.