Probably built up scale and the normal cooling of the heat exchanger and tank during the night. The heat exchanged in a gas water heater is often a metal spiral running up the middle of the tank, and held in with just a few welds at the top and bottom. As it cools and heats it shrinks and expands and rubs on the tank wall creating all sorts of odd noises. It may become stuck, and make a single larger movement that can be relatively loud. You may even hear sizzling from rain or condensation in the flue falling onto the hot heat exchanger. The scale in the bottom of the tank and on the heating column can make all sorts of pops, grinding noises, an occasional squeal, etc. as it changes size with temperature, breaks off, and then falls to the bottom.

My Rinnai gas tankless water heater has a built in recirculation feature. My furthest bathroom is 50' from the tankless. The shower is hot within 8secs of turning it on. My recirculation pump is on a timer and is programmed based on my family behavior. I was really concerned after reading so many of the reviews online about tankless water heaters topics like temp sandwich and not being able to use multiple showers and appliances at the same time. I have low flow shower heads in my bathrooms from Grohe and Hansgrohe. Two of my bathrooms have thermostatic valves. The third has a very old pressure balance valve that will be replaced soon. The shower heads are rated at 2.5gpm. They perform really well. The tankless also has a feature to fill a bathtub,. I have used this once and the tankless worked like a champ and filled a 50gal tub in 9 mins. I have city water and will perform my first flush of the system in May. I was also afraid of the temperature drops as it got colder. We had a week of sub zero temps and I noticed no impact on the water heater. We can take two showers concurrently, run the dishwasher, washer and a faucet at the same time. My home has a crawl space and most of my plumbing runs through the crawl. The tankless unit is hanging on the wall in the utilities room. Just wanted to post another experience in tankless. One of the main drivers for going with tankless in my case was that I needed the space. The tank heater blocked access into the utility room and a wall needed to be taken down to remove the heater. I have a Rinnai RUR98i. Good luck!

Nevada, NW of Reno, and something between 1350 and 1400 sq ft. - Checked and it is 1394 sq ft according to the realtor listing. It is a single story ranch on a slab. I am at something between 5000 and 5200 feet in altitude. I looked the furnace up on the Bryant website and it is 80% efficiency but I don't know which of the listed BTUs on the label should be used to determine the proper size of the furnace. The ceilings are insulated but the walls have little or no insulation to my knowledge. The calculator to which I was referred gives a result of approx 70k BTU. So if the highest listed BTUs on the label (88k) is the correct one to use for the size calculation, that results is 80% of 88k = 70,400 BTUs - so it would seem the furnace is properly sized. If that is NOT the correct value for BTUs, all the others listed on the label are less than that and the furnace must then be too small. We do normally get a lot of solar exposure here which may or may not enter into things. I have never set the heat higher than 68F and it is usually set to 60F at night and whenever I am gone. I'm on a fixed income so I watch those bills like a hawk and just wear more clothes. I did find a registry to look the business up as far as licensure, and it states he is licensed and the license is active. Keep in mind there is no AC so until it got quite cold here the furnace - or more correctly the blower - was not being used. I turned it on sometime in late Oct or early Nov. So before that there was no fan noise because the unit was not being used at all. The furnace is centrally located so that duct runs are shorter than usual. These houses were military housing for Air Force Brass and whatever else you could say about them, the floor plan and layout are quite efficient.

Found a fix!!! Its getting to about -30 here this week and the clicking on the exhaust outlet is back! It sounds like its from inside but I earned its happening outside, the sounds just echoing through the pipe. I had the same annoying 'clicking ticking sound as my furnace ramped up. The sound slowed down as the furnace ran, then when the furnace stopped, it 'clicked' again and again until the exhaust was cool. I sat outside and watched the cap as my furnace ramped up, the sound came from the 'fins' on the exhaust cap against the larger breather pipe. As the breather warmed from the exhaust in the inside pipe it expanded, causing friction against the fitted cap fins. I could actually see the fins' click as they shifted slightly. To fix I removed my exasust cap, then simply filled down the fins slightly where they fit so snuggly to the breather pipe. I reinstalled the cap, and no more clicking!! Win! Was so frustrated, I tried everything else before this You can see the small gap I filled down, this allows the breather pipe to warm and expand but not force against the fins to tightly. I did not change anything else, just removed cap, filled the fins a bit and reinstalled. Hope this helps, Ash