I leave my furnace fan run constantly, furnace or air. I notice that there are intermittant surges happening some for a second some longer and there is no pattern. The fan just surges but doesn't shut off. The rush happens then disapates and then happens again over and over. What do you think is causing this?? Is the fan motor going to break, is it belt? Should I call a technician. This isn't new, its been happening but no it is more frequent! Carrier furnace about 15 years old, forced air, in basement. Air works, heat works but its this fan issue and its below the bedroom so it is very audiable.
HELP

If it is a belt drive motor blower it could be a belt.

Is the air filter clean?

Sometimes the drawing for the electrical diagram will come off the blower door and get stuck in the blower wheel. Turn off power and check for that.

Could also be the blower motor run capacitor.

If all else fails call for service.