For what it's worth there's a thread about making the older Suburban furnaces quiet like the new ones.
It involves different fan blades and either electronically slowing the currnet motor for $5 or replacing the motor with a new slower one and the cost is $80.
But it will take a couple of hours to remove the furnace
, install the new motor and reinstall it.
But the same time would apply for installing a new furnace
plus the price of the furnace
Here's your fan motor for $78.05 plus $9.99 shipping
Suburban 520949 Furnace Motor for NT12 NT16