Probably non-metallic piping which has been firmly clamped against a wooden framing member (big mistake). The piping changes dimensions with changes in temperature--expanding when heated -- no power on Earth can prevent it from doing so. It slips in the clamp and each such movement produces the chatter that you hear. It may gradually lessen over time with wear on the pipe and clamp. Or open the wall now and correct the problem. Classroom study of chemistry and physics is helpful in understanding materials and accommodating to the characteristics of those materials.

Do you have an expansion tank? And do you have a Pressure Reducing Valve (PRV). Both the tanks and the valves often give out after about 10 years. I started getting weird noises in my water system after my house was about 8 years old. I checked the water pressure downstream of my PRV and it was high, and couldn't be adjusted downward. I got a PRV rebuild kit and was once again able to adjust the pressure. I then checked the expansion tank, and that wasn't functioning properly either, so I replaced it. The result was proper water pressure in the house and no more strange noises. I don't know if this is a fix for you, but it's one possibility.

My last water heater lasted 16 years. Personally, before I spent a bunch of money on a new heater, I would try a quick cleaning. It is actually MUCH easier than that article makes it seem. It is the same way you would clean a coffee pot, but on a much larger scale. Dump out the junk, rinse it clean, hit it with a cleaner, and rinse again. Basically, you just turn off the water heater, shut off the water, turn any faucet in the house to hot, and then drain the tank out the bottom. The water coming out will likely be a gunky mess. If so, close the drain up, turn the water back on for a minute, turn the water off, and drain it again. Repeat as long as you are getting gunk out instead of clean water. Pour in some vinegar, let it sit for a few minutes. Add some water, drain. Add some water, drain. Fill it back up with water and turn the heat back on. Depending on how bad the problem is, you may be able to fix it for $5 of vinegar and a couple hours time.

Water service pipe appears to be 3/4". A 3/4" trunk goes to the boiler, and everything else that branches off is 1/2" copper. How do I find out the second question's answer? It's city water... should we call the water utility company tomorrow? From street to house is aprox 50 feet, with the street main higher in elevation by about 10 feet. The water main comes into the "old part" of the one-story ranch house (built 1956) and branches off in old-fashioned trunk-and-branch to the following: kitchen: one sink, one bar sink, one DW, refrigerator ice maker hall bath: tub/shower combo, toilet, lav laundry: front-load washer outdoors: two spigots master bathroom (new, what we're discussing): 60 x 32' soaking tub, shower with regular showerhead plus handheld (two valves), two lav sinks, toilet. basement: 80 gal water heater All fixtures are on the first floor, although we're considering running the supply pipes to the master bath up into the attic and back down. Actual elevation of the house is just a bit above sea level... do you need the real elevation in feet? wisehvac, I believe the PEX we have been using is Zurn. From the big box stores.