Rumbling sound?

I sometimes hear a rumbling sound from under the hood of my 2001 Montana. It seems to go away when the heater is turned off. I suspect the blower motor, any suggestions?

I had a similar problem that turned out to be dust and debris sitting on top of the blower fan, such as a leaf and small stone. What I did was I removed the cabin air filter in the back of the glove box, and with the power OFF stuck my hand in and down. You should be able to feel the fins and the mount (it will feel like a cone). As well you can feel the blower fan to see if it is loose. This might help a little bit.