WhisperGen is the one I was thinking of.

I went down a rabbit hole on that - apparently the WhisperTech factory got destroyed in the New Zealand earthquake and left little behind other than a cohort of dedicated fans keeping old units alive :(

  How It Works

  The WhisperGen PPS16 is based on a four-cylinder Stirling-cycle (external combustion) engine that repeatedly heats and cools a mass of pressurised nitrogen gas.

  Each time the gas is heated and cooled, the changing gas pressure causes the pistons to move up and down.
  This mechanical motion, via a special mechanism called the “wobble yoke”, rotates an alternator to generate DC electricity which can be used to charge a lead-acid battery bank.

  The nitrogen gas is heated by a continuous-combustion burner, and cooled by coolant circulating through engine cavities. Heat transferred to the coolant can be used to heat domestic water cylinders and for space heating.

There was a European branch in Spain, seems to have withered, and some stories about expansion plans and pivoting that got trashed by the earthquake fallout.

~ https://www.smh.com.au/business/nzs-whispergen-to-move-to-eu...

Damn shame - I had access to an early model looong ago and was really looking forward to seeing where that went.