My furnace fan makes a struggling noise when on heating mode only.
It sounds like the fan has a hard time starting.
When on air conditioning mode it doesn’t make that noise; the fan sounds fine.

If the fan motor is having a hard time running you might try replacing the fan motor run capacitor with a like same new capacitor to see if that helps. This is the cheapest cure to try first. Get the same value of capacitor as what is in the unit now.