rickr,
I hate to tell you this, but from your description it does appear that you have a permanent split capacitor 1 phase motor and you will not be able to use an electronic soft start device with it. The soft start components would be damaged by the capacitors in the motor. This will also prevent you from using a VFD as well, even if you found one that works on 1 phase motors. I have in the past used a soft start on a temporary fan with full knowledge that it would not last long, but it made the problem go away for a few months anyway. Baldor makes a cheap 1 phase soft start controller that you can simply mount and wire downstream from whatever is controlling your motor now. It may only last a few months, or you might get lucky and it will last a year or so, you never know. Your only alternative electrically is to change out the motor to 3 phase and add a VFD which will convert your 240V 1 phase power to 3 phase for the motor. FDigure $300-500 plus labor minimum.
Another approach would be a "mechanical soft start" clutch device such as a Torque Tamer, shot coupling or fluid coupling that will slowly allow the blower to accelerate even though the motor is started at full speed rapidly. Contact a Power Transmission distributor such as Motion Industries or Kaman Industrial for advice.
That pop you here is probably the sheet metal or even the fiberglass duct "oil canning" as busbar suggested, so another option may be to solve the problem mechanically, perhapse with spray-on sound dampening material or something like that.
Good luck. Subvert the dominant paradigm... Think first, then act!
