I have searched the community and haven't found any reference to one. Like someone else a while back I want to know when the HVAC fan turns on. I have ecobee but of course due to polling it can take a minute to get the fan status. I would like to turn up the TV in my office when the fan comes on, turn it down when it goes off. I'm right next to the HVAC and it's a first world problem for sure, but there it is. Seemed like a noise sensor would part of my solution, but I can't find one. Any pointers? Thanks!
Jay
An easier way might be a shelley relay inline on the fan circuit to monitor power draw from the motor and use that to initiate the trigger. (If I wanted to do it I would simply use my iotawatt do make the notification, only because I already have one). Another option may be a vibration sensor on the fan housing.
I guess I have to buy a vibration sensor and see if it will work. I think it will need to be pretty sensitive as it doesn't seem to really vibrate much.
Shelley relay would also work...
Looking into that now. Totally unfamiliar with them. Seems to be WiFi based. Assuming it be passive, just letting you know when power is flowing.
You can configure them active or passive. Yes wifi, but 100% local no cloud and native integration into hubitat. If you want to stick with z-wave you could use a zooz zen-16 or 17
If you switch to something like a Honeywell T6-Pro Z-wave You could use attributes from that to fire the rule (I prefer those way over ecobee)
Hmmm. I will think about that. I actually like ecobee a lot, have 4 extra sensors, and the WAF is very high. Plus, it's just darn pretty on the wall.
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I have 36 Window sensors, all of which have Temp as an attribute. I can aggregate them all or just some of them for thermostat control if I want 
I don't right now but could. They're amazing little sensors and cheap AF...
What would a noise sensor be measuring, in a way that discriminates the noise you want to cause it to trigger vs. all the other noises you presumably wouldn’t want to cause it to trigger?
In other words, how would the sensor know, “this noise is the hvac fan kicking on, trigger. But that noise is a fire truck passing by outside, do not trigger?”
Glass break sensors and fire/smoke alarm listeners, both of which I have used, trigger based off of a much more specific pattern of sounds.
Would a decibel meter and rule logic like, “if x decibels for y seconds (or minutes), then trigger” maybe work?
I think @rlithgow1 is right that another type of sensor might work better for your application.
But I’m hardly a noise sensor expert, so I’m curious 
.
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You obviously don't have young children 
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Thanks for asking. Well, there's really no other loud sounds in the utility closet which is in the basement that would be obvious. My geek den (office) is in finished space right next to the utility room. When the fan kicks on it is LOUD. Loud enough to be annoying though I admit I'm sensitive to that kind of noise. Sound isolation/insulation takes you only so far (I've done it) because most of the remaining noise goes though the vents. I've even built a sound baffle in the vent leading to my office. I thought I would put the noise sensor right next to the fan and I think it would be obvious that it's on - above a certain DB level. I'm hesitant to introduce the shelly relay into the equation, probably just being chicken as I have done many other more complex things. Just messing with the HVAC though gives me pause.
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Tada
I built a custom little board that I use to monitor the wiring from my Ecobee thermostat to my HVAC system. IIRC, these wires carry 24 VAC, so I had to build a circuit to convert that AC signal into a DC signal (without affected the HVAC system operation!) so that an ESP8266 digital input could determine exactly what the thermostat is calling for (i.e. Mode Heat/Cool, Fan On/Off, Compressor On/Off, Auxiliary Heat On/Off.) It has worked great for a few years now, and of course the data is real-time (no polling, no cloud, etc...)
Probably a little more complicated than what most folks a re looking for. 
Another solution is to use a Current Sensing Switch, like the following, to determine if the fan motor is running. Attach the outputs of this device to a Contact Sensor with external wiring contacts, and you've got a very simple solution.
https://www.amazon.com/Current-Sensing-Normally-Adjustable-SZC23-NO-AL-CH/dp/B07P87TG8F
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I’ve been using this solution for over 5 years. It works very well, and is “non-invasive”, in that you don’t have to work with any high-voltage wiring. Especially if you get a split-core current sensing switch.
My Hubitat automation raises and lowers Sonos volume based on the state of the HVAC blower.
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BINGO. My use case. I think I'm going to go this route. Would you mind sharing which current sensor and contact sensor you are using?
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Only one I have found. I recently had to replace this, with a newer one, but the tilt sensor is what failed. So it should be ok. Wish I hadn't tossed it just recently...
https://smile.amazon.com/Ecolink-Intelligent-Technology-Z-Wave-TILT-ZWAVE2-ECO/dp/B00HGVJRX2/ref=dp_fod_1?pd_rd_i=B00HGVJRX2&psc=1
The split core current sensor switch was from Functional Devices. The model is the RIBXGTA (RIBXGTA - Functional Devices, Inc.).
And the contact sensor is an old GoControl contact sensor. The Ecolink you’ve picked will work at least as well.
I use an adjustable current sensor switch to prevent the volume change when the fan comes on at a low speed for air-circulation, but it comes on when there’s a call for cooling or heating. My blower has an ECM motor.
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Thanks much to you and the collective wisdom of this group. I've got a direction now.
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In his defense, he did say "noise sensor" and not "noise maker" expert 
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