This just started on my son's 06 Vibe. The blower motor does not start at times, and now when it does start, it grumbles just like you said. I'm planning to pull the blower motor and take it apart to see if I can fix it. There is a thread on here where several members have done that and fixed theirs so they did not have to buy a new motor.
http://forums.genvibe.com/zerothread?id=26737If you want to get a used one, they are quite a bit cheaper than new. There are several of them in the DFW area between $20-45. Just look for a Vibe blower motor on
http://www.car-part.com and put in your zip code & Texas for the search and you will find several listed.Good luck!