does anyone have any idea what’s going on with this wall furnace? This was on a coastal San Diego condo. The exterior temperature was approximately 60°, not very cold. The rattling started after the furnace had been running for about 20 minutes and continued after shutting it down. First time coming across something like this.
Video: Rattling Furnace
Was it windy that day?
No sir, just a light on shore breeze
Well, huh, whatever qualified professional you referred it out to will figure it out, eh?
I think you broke it. 
Did you check for Recalls.
These wall heaters have a long history of issues, and many have recalls associated with them.
Did you check the oil?
Sounds like it’s got a knocking rod. 
Calling that a “rattle” is quite generous…. sounds like gun shots.
Probably the blower fan has something caught in it or has failed bearings and is flopping around.
Did the furnace have a blower?
Someone, or something, is trying to get out of that thing. 
The seller agrees. They have never heard it.
The seller stashed the kiddies in the unit so they would not get in your way. Apparently they did not realize you would be testing the system. Probably the little kiddies in there whirling around.
Just an opinion but, a damaged heat exchanger can produce rattling or popping noises as the crack in a heat exchanger expands and contracts with temperature changes.
That’s my wag.
Total guess, but it sounds almost like a continuous backfire. Something in gas valve could be causing it.
Farewell and adieu to you fair Spanish ladies…. Recommend a Bigger Boat… https://youtu.be/hSbbc06EW9g?si=TVCAlNO1AMb2SkXW&t=32
Or the cat.
Damn gremlins get stuck and raise hell trying to get out.
It could be several things, but obviously, report the noise for repair by a HVAC tech. Try to talk to the tech after it is fixed (or maybe replaced).
Fan. Open it up and look next time.
Curious…
Did you bother checking for a Recall, (Manufacturer unknown to us on the MB), or did you just blow off my comment like many here do??
Hopefully your clients don’t die from…
Maybe you need all caps in Bold 2" font!
