OK there Little pellet piggy
We have the same stove model and it does the same thing.
The issue is the Triac on the control board is getting a tad flakey.
Each feed setting allows the room air blower to run within a certain speed range.
The Triac is a device that chops off a portion of the 120V wave form (AC Voltage cycles above and below the zero line)
If the triac starts to get goofy and the wave form starts to get out of whack a little, then as the feed motor cycles the voltage on the board changes some and then the fan reacts to the change in the wave form.
The speed of the fan is controlled by how long the power is actually on on each phase of the cycle.
Sort of like flipping a light switch on and off really fast. (many many times a second.
Now, short of replacing the board the only trick I have found is to adjust the fan speed to a spot just below the 1/2 way mark and tweek with it slightly until the pulsing stops.
Once you find the sweet spot, leave it alone and dont mess with it.
Ours has been set in the sweet spot for some time and it works fine. The airflow is plenty for a higher heat setting as well as the low setting.
As long as the thing works, your set.
The choice is to replace the board or go to a whole different controller on the stove and then run the Blower with a dedicated triac speed controller.
These control boards are not all that Heavy duty and will over time develop this anomally.
The cost to replace the board is not cheap ( unfortunately the Mfg cut corners on these boards at the time they were built)
There are a couple things that can cause the board to go south. The fan can have issues as can the feed motor, this in turn places more stress on the boards components.
Try finding the sweet spot and you should get rid of the surging noise.
The original design was to have the Blower speed increase with the higher heat setting.
The designers were of the opinion that the average Joe user was not smart enough to increase the fan speed a little when they increased the heat output. ????????
Probably, they were right. Far too many folks dont want to RUN their stove.
Earthstove had a simple solution and it was about as bomb proof as you could make it.
The room air blower was on a variable speed controller (Knob) and it went from Off to full speed.
If the operator did not sselect and speed that was adequate, as soon as the stoves hot air plenum reached a preset temperature, a snap swich closed and turned the fan on high speed until the stoves temp dropped back down.
The system used by Whitfield in these older units was just too busy with too many functions taxing the controls.
As the heat output knob was rotated to a higher setting it gave the fan speed controller more to work with.
So on number1 setting you had X and as you increased the fuel feed the fan could go to Y
Sort of hokie but when new it worked fine.
Snowy