Hang a cordless drill on the wall and tape the trigger at about 1/3 speed and that's about as bad as it gets with the fan. Wrap on the drill with a screwdiver swinging a few inches and hitting it with the plastic handle end and that's about the worst you will hear with the gas valves.

I tried the drill test, and it's way to audible to go where I had intended, inside my garage. I don't think any level of additional isolation/insulation attempts will help it either.

I have a position I can outdoor mount it, but it's going to be about 12" from my daugther's wall in that position as well. I didn't have time to do the drill test here, but I'll do that today and see how it sounds. If that doesn't work, I'm probbaly just going to have to end up sticking with a tank.

I don't find mine any noisier than a tanked model. Yes there's a bit of soft "whump" when the burners alight, but that's probably below 50 dB, or the sound a very quiet dishwasher makes. (A library is around 54 dB.)

Based on my experiences with a direct vent tanked model, a direct vent model would probably be quite a bit noisier. The direct vent tanked model was more on the order of 75-80 dB.

My current HWH is not direct vent. The only noise it makes is when the burner lites, which is just a quickj seond "whoosh", and then the sound of just running gas. There is no motor to make additional noise. So, unless you are standing 1 foot or closer, it's silent.

I'm going to call up the plumber I've been talking too and see if he can get me to some kind of demo house so I can hear it myself. That's the only way to know for sure.

Tankless water heaters under full fire range from 75-90 decibels @ 3M, depending on brand, model and venting. Although it is rare that very often it is at full fire, so it can be less. This can easily be simulated with a decibel meter and a variable speed drill I mentioned earler. BTW, if you have an iPhone, you can download a free app called "Sound Level" that is pretty accurate.

If the water heater is in an other area, typically it isn't the sound that it makes obnoxious but the transmission of sound through the walls that it is hung on, besides the gas line and venting and their capability to transmit sound and vibrations, when touching other items.

Many steps can be taken to reduce this such as full port gas flex lines, vibration isolators for the heater and making sure venting is binding.

Second test done today, this time with drill NOT touching the wall. Couldn't hear it this time. So it jives with the vibration transmission through the wall you speak about.

So, question is, are there any "standard" methods for reducing the transmission of noise through the wall for tankless HWH installs, or is it all on-site engineered stuff?

PS - My house is re-piped with PEX, it all runs overhead in the attic and then down through the walls. The PEX is loose inside the walls

Vibration isolators are commercially available but expensive, difficult to find, especially in the vertical mount type.

Perhaps in your case instead of mounting to wall, set 2 studs floor to ceiling not touching wall that connects to home (so 1" off wall maybe) cover with plywood and mount tankless.

zl700 I'm really wondering which brands/models you have experience with that are so noisy? Where does that noise you are describing originate? Are they vented through a flue straight up through the home or are they direct vent through a wall? We were thinking of adding a second tankless for a master bath expansion, but if the new ones you've experienced are so noisy, I don't think I want one located close to where I sleep.

I've lived with a tankless for going on 14 years now, and have never had anywhere near the level of sound that you are describing. Mine is in my laundry room about 5 feet away from the washer and when I do a hot water only wash, the washer filling makes more noise than the water heater does. As I said, you can hear a soft "whoomp" as the burners light, and a bit of water flow noise through the pipes, but it's barely audible standing at the washer 5 feet away. There is no gas pipe noise. I've never even heard gas piping make any real noise and we've got gas heat and stove. In fact, my gas fired forced air heater in the attic makes more noise when the burners light than the water heater does---not counting the sound of the air through the ducts. My water heater is a Myson, which I don't know if it's still made as it's an old fashioned design without fancy computerized controls. But it's worked very well for almost 14 years now--and with very little noise. :) I'll probably do a search and see if I can find another one if all of the newer brands aren't as quiet.

They are all about the same, when comparing BTU's.
However the 2-pipe (sealed combustion units) tend to be quieter because there is no inlet openings on the cabinet for combustion air, thus a bit quieter since some of the operation noises are contained.

My first tankless was a CEC Aquastar 170, gravity vented, non-powered, and the only noise was the whoosh of the burners. We are now speaking of draft induced or power vented tankless units. Many publish there DB ratings and 85 to many people would be loud. As I said earlier it doesn't always get to the point but it can.
In my homes Ive had a Noritz, 2-Rinnai, and 3-Takagi's, and 1 A.O. Smith power vented tank, the tank heater by far was much noisier.

The gas pipe I referred to is not the gas but the transmission of noise via vibrations and harmonics caused by the fan, and the snapping of the staging of the gas solenoids.

If your Myson is 14 years old I'm betting it has a modulating hydraulic gas valve and atmospheric vent so it would be virtually silent. And if that's the case, they weren't touted to be very efficient over a tank with exception to standby loss which was eliminated but sold as continuous hot water and space saving.