often bus heaters are starting to get scrappy by the time we get them..
my bus is 25 years old so maybe mine were worse than normal.. but almost all my fan motors were dirty and getting noisy bearings, my main heater core was corroded and leaking... I wouldnt have wished that thing on anyone. I tore it apart and rebuilt it..
with so many low-cost small profile hot-rod heaters out there, the old big bergstrom bus hetaers dont have a huge market unless say a church or actual school needs one for a bus that is still in passenger service..