Home heating system question

Question

My heater had it's twice yearly service about a month ago and everything checked out. I used the heat for the first time a few days ago and it made a loud noise (like a lawn mower). I called the service company and the technician said that the exhaust fan motor is out and needs replaced.

I find it odd that this wasn't caught during seasonal service visit. Is it possible that the technician didn't put the unit back together properly or something similar that could have caused this? The system is a Trane and less than 8 yrs old.

Thanks!